Method and apparatus for enhanced photo-thermal imaging and spectroscopy

ABSTRACT

System for performing chemical spectroscopy on samples from the scale of nanometers to millimeters or more with a multifunctional platform combining analytical and imaging techniques including dual beam photo-thermal spectroscopy with confocal microscopy, Raman spectroscopy, fluorescence detection, various vacuum analytical techniques and/or mass spectrometry. In embodiments described herein, the light beams of a dual-beam system are used for heating and sensing.

PRIORITY

The present application is a continuation of U.S. patent applicationSer. No. 16/465,824 filed May 31, 2019, which is a National Phase entryof PCT Application No. PCT/US2017/063807, filed Jun. 7, 2018, whichclaims priority from U.S. Provisional Application Ser. No. 62/427,671,filed Nov. 29, 2016, 62/505,533, filed May 12, 2017, 62/540,008, filedAug. 1, 2017; 62/541,749; filed Aug. 6, 2017; 62/567,037; filed Oct. 2,2017, and 62/569,944; filed Oct. 9, 2017. The disclosures of each of theprovisional applications and the PCT application are all herebyincorporated by reference.

BACKGROUND 1. Field of the Invention

The specification relates to investigating or analyzing materials by theuse of optical means, i.e. using infra-red, visible, or ultravioletlight. Embodiments described herein relate to imaging and spectroscopy,and, more particularly, to enhancements to photo-thermal imaging andspectroscopy systems and techniques for acquiring spectral informationindicative of the optical properties and/or material/chemicalcomposition of a sample, for example, information that correlates to aninfrared (IR) absorption spectrum.

2. Background of the Invention

Fourier Transform Infrared (FTIR) spectroscopy is the most common formof IR spectroscopy. FTIR works by measuring transmission of an infraredlight through a sample or reflection of IR light from a sample as afunction of wavenumber (a measure of the frequency of the IR light).FTIR based microscopes combine an FTIR spectrometer and microscopeoptics to provide spatially resolved measurements of IR absorption,transmission, and/or reflection. Conventional FTIR microscopy can onlyachieve spatial resolution on the order of the wavelength of the IRlight. The fundamental limit is determined by optical diffraction and isset by the wavelength of the IR light and the numerical aperture of theIR illumination and/or collection optics. Practical limitation maydegrade this spatial resolution further. The spatial of the FTIRmicroscope is wavelength dependent, but is on the order of 10 microns intransmission for wavelengths in the mid-IR. An example of an FTIRspectroscopy approach is shown, for example, in U.S. Pat. No. 7,630,081,which describes recent improvements to FTIR interferometers. FTIRspectroscopy can be insufficiently precise, and involves significantsample preparation to insure appropriate transmission of the mid-IR beamthrough the sample, which is not practicable or desirable for manyopaque, frangible, or biological substances.

Attenuated Total Reflection (ATR) spectroscopy is based on indirectreflection of a beam through an intervening crystal in direct contactwith the sample. ATR spectroscopy can only achieve resolutions on theorder of 3 microns using mid-IR beams. Unfortunately, ATR spectroscopynecessarily requires direct contact of the intervening crystal with thesample which can cause deformation or breaking of the sample, andrequires a significant amount of sample preparation, particularly fororganic samples. Furthermore, reflection or refraction between thecrystal and the sample requires good contact between the two. If goodcontact is not established, then the light may reflect or refract basedon the refractive index of the material between the sample and thecrystal, rather than based on the properties of the sample itself.

Raman spectroscopy is based on measurement of Raman scattering mresponse to illumination of a sample by a mid-IR beam. Ramanspectroscopy can achieve resolutions as low as a few hundred nanometers,but usually has a practical limit of 1 micron or more. An early exampleof a Raman spectroscopy approach is shown, for example, in U.S. Pat. No.2,940,355. Although Raman spectroscopy can achieve resolutions down toseveral hundred nanometers, the information generated from this approachincludes noisy or dispersive artifacts that are inherent in Ramanscattering.

U.S. Pat. No. 9,091,594 describes an alternative non-destructiveapproach for photo-thermal spectroscopy for chemical spectroscopy andimaging that uses two beams of light of differing wavelengths to achievesub-micron spatial resolution, but in a non-contact manner and withoutthe onerous sample preparation requirements associated with ATR or FTIRtechniques described above. The method describes illuminating a samplewith a first beam of IR light having a wavelength of at least 2.5microns to create a photo-thermal change in a region within the sampledue to absorption of energy from the first beam, and then illuminatingat least a portion of the region within the sample with a second beam oflight having a wavelength of less than 2.5 microns to detect thephoto-thermal change in the region at a resolution smaller than adiffraction limit of the first beam.

Although the alternative dual beam photo-thermal spectroscopy techniquedescribed in U.S. Pat. No. 9,091,594 provides significant advantagesover the three general approaches to mid-IR spectroscopy and imaging,further enhancements and improvements to this new photo-thermaltechnique are desirable.

SUMMARY

Various embodiments are described for performing chemical spectroscopyon samples using a multifunctional platform that combines the analyticaland imaging techniques of dual beam photo-thermal spectroscopy withconfocal microscopy, Raman spectroscopy, fluorescence detection, variousvacuum analytical techniques, and/or mass spectrometry. In embodimentsdescribed herein, the light beams of a dual-beam system are used forheating and sensing.

The dual-beam system as described in various embodiments can include atleast a first light beam of infrared radiation for heating and aseparate second light beam for probing/sensing having a wavelengthshorter than the first beam. In various embodiments, these light beamscan be arranged parallel to one another, or arranged along a commonpath, and these two different beams may be spatially distinct from oneanother or may be distinguished by different wavelengths.

The heating/infrared beams can be, for example, a beam that is tuned toinduce molecular vibrations in a sample. The probing/sensing beams canbe a beam that is tuned for high resolution detection of thecharacteristics of the sample, and can have a lower wavelength than theheating beam in some embodiments. Characteristics of the sample that canbe measured by the probing/sensing beam include index of refraction aswell as deformation, expansion, and/or change in index of refraction ofa sample. In some embodiments, the probing/sensing beam can also detectproperties of the sample, such as by Raman spectroscopy, fluorescence,or combinations of those techniques.

For the dual-beam photo-thermal spectroscopy, a sample region isilluminated by the heating light beam and the resulting photo-thermalresponse due to infrared absorption is read out as an undistortedspectra with the probing/sensing light beam. The measurements collectedby these two light beams operating in coordination with one anothercontains more data and can be collected at a higher resolution thanoperating the beams independently from one another. That is, theprobing/sensing light beam has a more precise spatial resolution thanthe heating/infrared light beam due to its lower Abbe diffraction limit,whereas the heating/infrared light beam can initiate spectroscopy dataor wavelength shifts due to a photo-thermal response that would not becaused by a sensing light beam operating in isolation.

Definitions

For purposes of this specification, the following terms are specificallydefined as follows:

Optical property” refers to an optical property of a sample, includingbut not limited to index of refraction, absorption coefficient,reflectivity, absorptivity, real and/or imaginary components of theindex refraction, real and/or imaginary components of the sampledielectric function and/or any property that is mathematically derivablefrom one or more of these optical properties.

“Illuminate,” “Illuminating,” and “Illumination” mean to directradiation at an object, for example a surface of a sample, the probetip, and/or the region of probe-sample interaction. Illumination mayinclude radiation in the infrared wavelength range, visible, and otherwavelengths from ultraviolet to a millimeter or more. Illumination mayinclude any arbitrary configuration of radiation sources, reflectingelements, focusing elements and any other beam steering or conditioningelements.

“Infrared light source” refers to one or more optical sources thatgenerate or emits radiation in the infrared wavelength range, Forexample it can comprise wavelengths within the mid-IR (2-25 microns). Aninfrared light source may generate radiation over a large portion ofthese wavelength sub-regions, or have a tuning range that is a subset ofone of the wavelength ranges, or may provide emission across multiplediscrete wavelength ranges, for example 2.5-4 microns, or 5-13 microns,for example. The radiation source may be one of a large number ofsources, including thermal or Globar sources, supercontinuum lasersources, frequency combs, difference frequency generators, sum frequencygenerators, harmonic generators, optical parametric oscillators (OPOs),optical parametric generators (OPGs), quantum cascade lasers (QCLs),nanosecond, picosecond, femtosecond, and attosecond laser systems, CO2lasers, heated cantilever probes or other microscopic heaters, and/orany other source that produces a beam of radiation. The source may benarrowband, for example with a spectral width of <10 cm⁻¹ or <1 cm⁻¹less, or may be broadband, for example with a spectral width of >10cm⁻¹, >100 cm⁻¹ or greater than 500 cm⁻¹. “Near infrared light”generally refers to a wavelength range of IR light corresponding to0.75-2 μm.

“Probe light source” refers to a radiation source that can be used forsensing of an optical property of a sample. A probe light source can beused to sense the response of the sample to the incidence of light fromthe infrared light source. The radiation source may comprise a gaslaser, a laser diode, a superluminescent diode (SLD), a near infraredlaser, a UV and/or visible laser beam generated via sum frequency ordifference frequency generation, for example. It may also comprise anyor other sources of near-infrared, UV, and/or visible light that can befocused to a spot on the scale smaller than 2.5 micrometer, and or evensmaller than 1 micrometer, and possibly smaller than 0.5 micrometer. Insome embodiments, the probe light source may operate at a wavelengththat is outside the tuning or emission range of the infrared lightsource, but the probe light source can also be a fixed wavelength sourceat a select wavelength that does in fact overlap with the tuning rangeof the infrared light source. A “probe light beam” or “sensing lightbeam” is a beam originally emitted from a probe light source. In someembodiments, the probe light source is selected to be a “narrow bandlight source,” as described below.

“Collecting probe light” refers to collecting radiation of a probe lightbeam that has interacted with a sample. The probe light can be collectedafter reflection, scattering, transmission, evanescent wave coupling,and/or transmission through an aperture probe.

“Signal indicative of” refers to a signal that is mathematically relatedto a property of interest. The signal may be an analog signal, a digitalsignal, and/or one or more numbers stored in a computer or other digitalelectronics. The signal may be a voltage, a current, or any other signalthat may be readily transduced and recorded. The signal may bemathematically identical to the property being measured, for exampleexplicitly an absolute phase signal or an absorption coefficient. It mayalso be a signal that is mathematically related to one or moreproperties of interest, for example including linear or other scaling,offsets, inversion, or even complex mathematical manipulations.

“Spectrum” refers to a measurement of one or more properties of a sampleas a function of wavelength or equivalently (and more commonly) as afunction of wavenumber.

“Infrared absorption spectrum” refers to a spectrum that is proportionalto the wavelength dependence of the infrared absorption coefficient,absorbance, or similar indication of IR absorption properties of asample. An example of an infrared absorption spectrum is the absorptionmeasurement produced by a Fourier Transform Infrared spectrometer(FTIR), i.e. an FTIR absorption spectrum. In general, infrared lightwill either be absorbed (i.e., a part of the infrared absorptionspectrum), transmitted (i.e., a part of the infrared transmissionspectrum), or reflected. Reflected or transmitted spectra of a collectedprobe light can have a different intensity at each wavelength ascompared to the intensity at that wavelength in the probe light source.It is noted that a IR measurements are often plotted showing the amountof transmitted light as an alternative to showing the amount of lightabsorbed. For the purposes of this definition, IR transmission spectraand IR absorption spectra are considered equivalent as the two data setsas there is a simple relationship between the two measurements.

“Modulating” or “modulation” when referring to radiation incident on asample refers to changing the infrared laser intensity at a locationperiodically. Modulating the light beam intensity can be achieved bymeans of mechanical chopping of the beam, controlled laser pulsing,and/or deflecting the laser beam, for example by a tilting mirror thatis driven electrostatically, electromagnetically, with piezo actuatorsor other means to tilt or deform the mirror, or high speed rotatingmirror devices. Modulation can also be accomplished with devices thatprovide time varying transmission like acousto-optic modulators,electro-optic modulators, photo-elastic modulators, pocket cells, andthe like. Modulation can also be accomplished with diffraction effects,for example by diffractive MEMS-based modulators, or by high speedshutters, attenuators, or other mechanisms that change the intensity,angle, and/or phase of the laser intensity incident on the sample.

“Demodulate” or “demodulation” refers to extracting aninformation-bearing signal from an overall signal, usually, but notnecessarily at a specific frequency. For example, in this application,the collected probe light collected at a photo detector represents anoverall signal. The demodulation process picks out the portion that isbeing perturbed by infrared light absorbed by the sample. Demodulationcan be accomplished by a lock-in amplifier, a fast Fourier transform(FFT), a calculation of a discrete Fourier component at a desiredfrequency, a resonant amplifier, a narrow band bandpass filter, or anyother technique that largely enhances the signal of interest whilesuppressing background and noise signals that are not in sync with themodulation. A “demodulator” refers to a device or system that performsdemodulation.

An “analyzer/controller” refers to a system to facilitate dataacquisition and control of the PTP system. The controller may be asingle integrated electronic enclosure or may comprise multipledistributed elements. The control elements may provide control forpositioning and/or scanning of the probe tip and/or sample. They mayalso collect data about the probe deflection, motion or other response,provide control over the radiation source power, polarization, steering,focus and/or other functions. The control elements etc. may include acomputer program method or a digital logic method and may be implementedusing any combination of a variety of computing devices (computers,Personal Electronic Devices), analog and/or digital discrete circuitcomponents (transistors, resistors, capacitors, inductors, diodes,etc.), programmable logic, microprocessors, microcontrollers,application-specific integrated circuits, or other circuit elements. Amemory configured to store computer programs may be implemented alongwith discrete circuit components to carry out one or more of theprocesses described herein.

A “lock-in amplifier” is one example of a “demodulator” (defined above)and is a device, system, and/or an algorithm that demodulates theresponse of a system at one of more reference frequencies. Lock-inamplifiers may be electronic assemblies that comprise analogelectronics, digital electronics, and combinations of the two. They mayalso be computational algorithms implemented on digital electronicdevices like microprocessors, field programmable gate arrays (FPGAs),digital signal processors, and personal computers. A lock-in amplifiercan produce signals indicative of various metrics of an oscillatorysystem, including amplitude, phase, in phase (X) and quadrature (Y)components or any combination of the above. The lock-in amplifier inthis context can also produce such measurements at both the referencefrequencies, higher harmonics of the reference frequencies, and/orsideband frequencies of the reference frequencies.

A “detector” in the context of the probe light beam, refers to anoptical detector that produces a signal indicative of the amount lightincident on the detector. The detector can be any of a large variety ofoptical detectors, including but not limited to a silicon PINphotodiode, a gallium phosphide photodetector, other semiconductingdetectors, an avalanche photodiode, a photomultiplier tube, pyrometer,bolometer, and/or other detector technologies that produce a signalindicative of the amount of light incident on the detector surface. Thedetector can also be fluorometers and/or Raman spectrometers.“Narrowband Light source” a light source with a narrow bandwidth orlinewidth, for example a light of linewidth smaller than 8 cm-1, but ingeneral it can be a light source with a linewidth narrow enough that thelinewidth does not cover a spectral range of interest of the sample.

“Raman” refers light that is inelastically scattered from a sample atone or more wavelengths that are different from the excitationwavelength due to Raman scattering. “Raman spectroscopy” refers tomeasuring the spectroscopic content (Raman spectra) of Raman scatteredlight, for example the intensity of Raman scattered light as a functionof Raman shift. “Raman spectrometer” is a device for examining Ramanshifts in light collected from a sample and producing Raman spectraand/or Raman images.

“Fluorescence” refers to the emission of light from a sample at onewavelength due to excitation at another wavelength due to fluorescentexcitation and emission processes.

“Mass spectrometer” refers to an apparatus for analyzing the molecularmass distribution of an analyte.

“Confocal microscopy” refers to a form of optical microscopy m which thelight collected at a detector is confined to light that passes through asmall volume within the 3D focus volume of an optical objective on asample. Confocal microscopy is often performed by placing a “confocalaperture” at a focal plane that is equivalent with the focal plane ofthe sample, thus blocking stray light that does not pass through thefocus volume on the sample.

“Vacuum Analytical Techniques” to any number of materialscharacterization techniques commonly or exclusively performed in vacuum,including, but not limited to: scanning electron microscopy (SEM),transmission electron microscopy (TEM), x-ray diffraction (XRD), energydispersive X-ray spectroscopy (EDS), time of flight mass spectrometry(TOF-SIMS), mass spectrometry (MS), and atomic force microscope-basedmass spectrometry (AFM-MS).

“Diffraction Limit” of a light beam means the minimum separation of twooptical sources that can be distinguished by a detector. The Abbediffraction limit d for a microscope having a numerical aperture NA andoperating at a wavelength λ is defined as d=λ/(2·NA). Physicalrestraints on the numerical aperture of a microscope prohibit very largenumerical apertures, and therefore the diffraction limit of a microscopedepends strongly upon the operating wavelength used for detection, withlarge wavelengths corresponding to relatively poor resolution and highwavelengths corresponding to increased precision.

The terms “about” or “approximate” and the like are synonymous and areused to indicate that the value modified by the term has an understoodrange associated with it, where the range can be ±20%, ±15%, ±10%, ±5%,or ±1%.

The term “substantially” is used to indicate that a result (e.g.,measurement value) is close to a targeted value, where close can mean,for example, the result is within 80% of the value, within 90% of thevalue, within 95% of the value, or within 99% of the value.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects and advantages of the embodiments provided herein are describedwith reference to the following detailed description in conjunction withthe accompanying drawings. Throughout the drawings, reference numbersmay be re-used to indicate correspondence between referenced elements.The drawings are provided to illustrate example embodiments describedherein and are not intended to limit the scope of the disclosure.

FIGS. 1A and 1B are simplified schematic diagrams of an embodimentsincluding capability to perform measurements of heating beam absorption,fluorescence, and Raman spectroscopy.

FIG. 2 is a simplified schematic diagram of an embodiment including beamsteering.

FIG. 3 is a simplified schematic diagram of an embodiment includingadditional details of the receiver module.

FIGS. 4A-4I are simplified conceptual diagrams of overlap betweenheating and sensing beams according to an embodiment.

FIGS. 5-7 are simplified schematic diagrams of embodiments within avacuum enclosure.

FIG. 8 is a simplified schematic diagram of an embodiment configured forwide area imaging.

FIGS. 9A and 9B are simplified schematic diagrams of handheldmeasurement system embodiments.

FIG. 10 is a simplified schematic diagram of an embodiment configuredfor measurements of samples immersed in liquid.

FIG. 11 is a simplified schematic diagram of an embodiment configuredfor measurements of a sample immersed in liquid.

FIG. 12 shows a simplified schematic diagram of an embodiment configuredfor use with a low frequency heating beam.

FIG. 13 is a simplified schematic diagram of a beam steering andautomated optimization system according to an embodiment.

FIG. 14 is a simplified schematic diagram of a portion of a PTP systemconfigured for selective masking of the sensing probe beam according toan embodiment.

FIG. 15 shows a simplified flowchart of an embodiment for substantiallyminimizing sample damage and/or spectral distortion while performing ameasurement.

FIGS. 16A, 16B, and 16C are simplified schematic diagrams of anembodiment configured to map the topography of a sample.

FIGS. 17A, 17B, and 17C are simplified schematic diagrams of a systemfor mapping sample height according to an embodiment.

FIG. 18A is a simplified schematic diagram of a topographical mappingsystem according to an embodiment, and FIGS. 18B and 18C are diagrams oftopography measured by the topographical mapping system of FIG. 18A.

FIGS. 19A, 19B, and 19C show example transmission spectra.

FIG. 20 shows a simplified schematic diagram of a combined sensing andheating system with an atmospheric sampling laser desorption massspectrometry system.

FIG. 21 is a schematic of a spectral cube according to an embodiment.

FIG. 22 depicts IR absorption across a cross-section of a samplemeasured according to an embodiment.

FIG. 23 is a cross-sectional view depicting locations of substances ofinterest in a sample according to an embodiment.

FIG. 24 depicts examples of three x-y scans corresponding to particularwavelengths/wavenumbers.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

Dual-Beam Imaging and Spectroscopy

The current disclosure is directed towards obtaining measurements ofoptical properties with a dual-beam system. The dual-beam systemincludes at least one heating beam, and at least one sensing beam. Thetwo beams are directed towards a sample of material and datacorresponding to that sample is collected at a resolution that issmaller than the diffraction limit of the heating beam. In embodiments,infrared absorption spectrum of a sample can be sensed with submicronscale resolution, and in some embodiments additional, complementarymeasurement techniques can also be used simultaneously or in parallel.FIG. 1A shows a simplified conceptual diagram of a principle of highresolution photo-thermal detection of infrared absorption. An infrared(IR) heating beam 122 is directed at a region of interest 102 of asample 125. If the IR light beam is tuned to a wavelength where thesample absorbs IR light, for example associated with a molecularabsorption band in the sample, then IR absorbing regions of the samplewill heat up. This photo-thermal heating can result in both thermalexpansion of the sample and/or a change in the index of refraction ofthe IR absorbing/heated regions of the sample.

These photo-thermal distortions in the sample can then be probed byseparate probe beam 127 configured for sensing other characteristics ofthe sample, generally with a shorter wavelength than the IR beam. Theuse of a shorter wavelength probe beam allows the probe beam to befocused to a smaller diffraction-limited spot than the IR beam. Thedegree of IR absorption can be sensed by measuring changes in thetrajectory of the probe beam 127 that is reflected and/or scattered fromthe surface (schematically indicated by arrows 110), and/or by probelight 132 that is transmitted through the sample. Specifically, when theprobe beam is focused onto a region that is absorbing IR radiation, theprobe beam can be affected by so called thermal lensing or other effectsto modify the intensity, phase, and/or angular distribution of the probebeam after interacting with the sample. As shown schematically in FIG.1A, for example the angular distribution of the probe beam can bemodulated (indicated by arrows 110 and 112) as the sample isperiodically heated and cooled via absorption of pulses of IR radiation.Thus by measuring changes to the probe beam as a function of IRradiation incident on the sample, it is possible to measure the IRabsorption properties of a sample, even down to sub-micron spatialresolution.

In alternative embodiments, other wavelengths could be used for theheating beam, such in addition to or in lieu of infrared. Additionallyor alternatively, one or more of the heating IR beam 122 and the sensingprobe beam 127 could be reflected from the surface of the sample 125,rather than being transmitted through the sample. These and othervariations are described in more detail with respect to otherembodiments below.

FIG. 1B shows a simplified schematic diagram of an embodiment employingthe detection mechanism outlined in FIG. 1A. A heating beam source 112emits a beam of heating radiation 114 towards a beam combiner 116 andthen towards optional mirror 118 and then to focusing optic 120 whichfocuses the beam of heating radiation 122 to a region of interest 124 ofa sample 125. The heating source 112 may be a tunable narrowbandinfrared (IR) source (e.g., an infrared laser or a broadband source).Focusing optic 120 can be a single reflective optic, for example aparabolic mirror or spherical mirror, any combination of reflectiveoptics, for example a reflective IR objective employing a Schwarzchilddesign (also called Cassegrain) or related design. It may also be apurely refracting objective capable of focusing IR and visible lightsimultaneously.

The sensing probe beam mentioned previously will be focused to a spotthat overlaps with the focused IR spot on the region of interest 124 ofthe sample 125. A sensing or probe beam source 126 emits a probe beam127 towards optional beamsplitter 128. Probe light 130 passing throughthe beamsplitter impinges on beam combiner 116 which then directs theprobe beam onto a path that is substantially collinear and/or parallelwith the heating radiation beam 114. In the configuration shown, beamcombiner 116 is substantially transmissive to infrared light andsubstantially reflective to the wavelength of the probe light beam 130.In an alternate embodiment, the beam combiner can be used in an IRreflecting configuration where the relative positions of the heatingsource 112 and sensing beam source 126 are reversed (i.e., the IRheating beam is reflected by the beam combiner 116 and the probe beam istransmitted through the beam combiner 116). In the first configuration,the beam combiner may be for example a longpass IR filter, for exampleas supplied by Edmund Optics. ISP Optics supplies metal film dichroicfilters that are also designed to selectively reflect IR and transmitvisible light or vice versa. Probe beam source 126 generally emits aprobe beam that comprises at least one wavelength that is shorter thanthe IR wavelength(s) emitted by IR source 112. In this embodiment thefocus spot size of the probe beam as focused by focusing optic 120 canbe substantially smaller than the smallest achievable focused spot fromthe heating beam, due to the difference in diffraction limit as betweenthose beams. For example, if the IR source emits radiation at 10 μm andthe probe beam is selected to have a center wavelength of 405 nm, theprobe beam can achieve a spatial resolution improvement (10/0.405)=24.6,i.e. almost 25× better spatial resolution.

To detect the IR absorption with this improved spatial resolution,embodiments described herein sense the changes in the probe beamassociated with the absorption of heating radiation. The probe beam iscollected after it has been reflected, transmitted or otherwisescattered from the sample. In the embodiment in whichtransmitted/forward scattered light is being sensed, the probe light 132that passes through the sample region 102-along with any light emittedfrom the sample due to the probe light illumination-can be collected bycollection optic 134. The sample may emit or fluoresce light and/orRaman scattered light due to the probe excitation. For the purposes ofthis application, “collected probe light” can refer to light of allwavelengths that are re-emitted from the sample after excitation by theprobe light beam and after interaction with the sample. As such, thesensed light can include light at the probe beam center wavelength aswell as light that is wavelength shifted due to fluorescence, Ramanscattering, or other optical processes.

The collected probe light can then be directed towards receiver module140, optionally being redirected and/or steered by optional mirror 138.In one embodiment, collection element 134 can be a high numericalaperture condenser lens, for example an optical microscope condenser, ora collection optic made for example by a high NA molded asphere, forexample sold by Thorlabs. The collection optic can also be a reflectiveelement, for example a spherical, aspherical, or parabolic mirror. Thecollection optic is shown collinear with the incident beam, but inalternative embodiments it can also be mounted off axis to capture lightthat is scattered over wider angles.

Receiver module 140 can comprise one or more detectors and/orspectrometers. For example, receiver module 140 can contain be any of alarge variety of optical detectors as described in the definitionssection, depending on the wavelength and intensity of probe light andthe bandwidth desired for a specific measurement.

The apparatus can also include additional filters, detectors, andspectrometers to enable simultaneous or sequential Raman spectroscopyand/or fluorescence measurements. In these embodiments, one or moreoptional fluorescence and/or Raman filters can be installed tosubstantially block light at the probe light source center wavelengthand pass light that is wavelength shifted from the probe wavelength.This approach allows detection of inelastically scattered light,including but not limited to Raman and fluorescently shifted light.Additional optional detectors can be used to detect and/or spectrallyanalyze the wavelength shifted light. Specifically, a Raman spectrometercan be used to measure Raman spectra from light that emanates from thetip-sample region due to the excitation by the probe light beam. Thereceiver module can be free space coupled and/or can have one or moreelements that are fiber coupled. For example, the collected probe lightmay be coupled into an optical fiber and then transmitted to a fibercoupled Raman spectrometer.

Samples may also be measured m a reflection/backscatter configuration.In this embodiment, probe light returning from the sample can becollected by the same optic 120 that was used to focus the incidentlight. In this embodiment probe light that is reflected and/or scatteredfrom the sample can be collected by objective 120 and returned along theincidence path, reflecting off optional mirror 118, reflecting off beamcombiner 116 back to beam splitter 128. A portion 129 of the returningprobe beam will be reflected to a receiver module 142. In someconfigurations, it is possible for receiver modules 142 and 140 to bethe same module. In one embodiment, at least a portion of the receivermodule can be moved from one location to the other, for example onself-aligning mounts.

Alternatively the receiver module may be broken up into two or moreseparate components, for example a fiber coupling component and adetector/spectrometer component. In this embodiment, fiber couplingcomponents can be placed at the positions shown for receiver modules 142and 140 and the fibers can be routed into a single remotedetector/spectrometer module, thus eliminating the need for duplicatedetectors/spectrometers. Although both a transmitted light path 132 anda reflected/scattered light path 129 are shown, either one alone couldbe employed as well. The embodiment shown employs an optical arrangementwhere the IR beam and probe beam are focused by the same focusingelement 120, (e.g., via a Schwarzchild objective). It is also possibleto counter-propagate the IR and probe beams, as described in“Super-resolution imaging with mid-IR photo-thermal microscopy on thesingle particle level” by Cheng et al, SPIE Proceedings Vol. 9549954912-1 2015, which is hereby incorporated by reference, in order to,for example, deliver the IR beam with focusing element 120 and deliverthe probe beam via a probe focusing optic in the position of 134, orvice versa. As discussed previously, the probe beam can be collected ineither reflection and/or transmission, depending on the opticaltransmission of the probe beam wavelength for the sample of interest. Itis also possible to deliver both the IR and visible light from the sameside of the sample, for example using one or more focusing objectivesinclined relative to the surface, as shown in incorporated referenceSer. No. 62/427,671.

The signal(s) from one or both receiver modules may be amplified usingcurrent and/or voltage amplifiers and then sent to analyzingelectronics, for example a demodulator or lock-in amplifier. The outputof heating beam source 112 can be modulated and/or pulsed using aninternal or external pulse control 146. In one embodiment, pulse control146 can be used to trigger a pulse emission from a pulsed IR lasersource or may be used to control an external modulator, for example ahigh-speed chopper or other beam modulator. Pulse control 146 can send atrigger or synch pulse to analyzing electronics 144 to act as asynchronizing reference. The IR laser is preferentially modulated and/orpulsed at a frequency ranging from the kHz to MHz regime or higher. Theanalyzing electronics then demodulate a strength of the variation of thedetector signal synchronized to the pulse control 146. In embodiments,pulse control can be generated by the analyzing electronics and used tocontrol the heating beam modulation, and can also act as an internaldemodulation reference as described in more detail below.

Because the collected probe beam signal is demodulated at a frequencycorresponding to the modulation of the IR beam, the demodulation signalcan be indicative of the degree of deviation of the probe beam inducedby the absorption of the IR beam at the sample. That is, thedemodulation signal is indicative of the IR absorption by the sampleregion where the probe beam is focused. As such the demodulation signalcan be used to map IR or other heating beam absorption of the sample onspatial resolution scales smaller than the diffraction limit of the IRbeam.

The demodulation signal can be measured at a plurality of wavelengths(or equivalently wavenumbers) corresponding to the heating beam sourceto obtain a signal 148 that is indicative of an infrared absorptionspectrum of the region 124 of the sample 125. These absorption spectracan be measured at a plurality of locations on the sample 125translating the sample relative to the focused IR and probe beams, forexample by moving the sample with sample stage 119, or by translatingthe objective 120, and/or by steering the IR and/or probe beams, forexample with beam steering mirrors described in FIG. 2 , or separatebeam steering mirrors after the beam combination optics. For example,mirror 118 or similar can be used to steer both the IR and probe beamsafter beam combination. In certain embodiments, additional scan lensesor other optics may be desirable. By measuring a spectrum 148 at aplurality of locations produces a family of spectra 149 that canrepresent the chemical/spectroscopic variation in the sample. Thespectra can be analyzed to produce chemical images 150 that show thedistribution of difference chemical species in the sample. Chemicalimage 150 can also be obtained by mapping the demodulation signal at afixed wavelength/wavenumber over a plurality of points on the sample.For example, the IR source can be tuned to a wavelength where at leastone chemical component in the sample absorbs. Creating a map of thedemodulation signal at this fixed wavelength as a function of positionon the sample can create a map of the distribution of the absorbingcomponent. Chemical image 150 can be created by tuning the IR source toa single wavelength and scanning over a plurality of locations of thesample and/or by measure IR absorption spectra at a plurality ofpositions on the sample and then analyzing the absorption at a singlewavelength or over a range of wavelengths. Additional chemometric andmultivariate analysis techniques can be applied to the family of sensedspectra to produce alternate compositional maps/chemical images.

One advantage of the various embodiments is that IR, Raman, andfluorescence measurements can all be sensed by collection of a singleprobe beam. As such it is possible to simultaneously or sequentiallycollect multiple chemical image maps 150 and 152, for example one map150 being an image of IR absorption and the other map 152 being a Ramanor fluorescence response image. This PTP apparatus makes it possible toobtain simultaneous or sequential measurements of IR absorption, Ramanscattering, and/or fluorescence over the same region of the sample (oroverlapping regions of the sample) for the purposes of correlativemeasurements. Furthermore, the measurements described above areperformed in a non-contact mode, and it is not necessary for anyphysical probe or crystal to create a mechanical contact with the topsurface of sample 125. This facilitates rapid, precise measurements thatare non-contact and accordingly non-destructive.

Spatial resolution. The spatial resolution that can be achieved can beset by one or more of (1) the size of the probe beam; (2) the area ofoverlap of the IR and visible beams; (3) the frequency of modulation ofthe IR beam. When the modulation frequency of the IR beam is high enoughsuch that the thermal diffusion length is much smaller than the spotsize of the probe beam, then the spatial resolution can be much smallerthan the IR beam spot size. The Abbe spatial resolution limit R isdefined as λ/NA as described above, assuming a perfect Gaussian beam andno aberrations in the focusing optics. For example, using a 405 nmwavelength probe beam to read out the IR absorption using an NA of 0.78,this leads to an achievable spatial resolution with the probe beam of260 nm. By comparison, the same diffraction limit at a wavelength of 10μm in the mid-IR would give a diffraction limited resolution of 6.4 μm,almost 25× coarser. As such the IR absorption profile of the sample canbe probed on much smaller length scales than the spatial resolutionlimit that would otherwise be constrained by the focus spot size of theIR beam. Even better spatial resolution can be achieved for example byusing ultraviolet radiation for the probe beam. In practice, it ispossible to achieve spatial resolution of less than 1000 nm, less than500 nm, and less than 200 nm, in embodiments. Using schemes to controlthe overlap of the IR and probe beams to less than the diameter of theprobe beam can be used to achieve spatial resolution better than thediffraction limit of the probe beam, for example less than 100 nm. It isalso possible to follow PTP measurements with atomic force microscopybased IR spectroscopy (AFM-IR) and/or scanning scattering near-fieldoptical microscopy to achieve measurements of IR absorption and/orscattering with a spatial resolution down to the scale of 10 nm or less.

FIG. 2 shows an alternative embodiment of the apparatus in FIG. 1B, andfurther depicts optional features configured to increase thefunctionality of the apparatus. The majority of figure labels are thesame as FIG. 1B and the associated description generally applies. Thediscussion of this figure focuses primarily on the additional features.

As in FIG. 1B, an infrared source 112 emits a beam of infrared radiation114. In this embodiment, optionally the IR heating beam is directedtowards two optional mirrors 200 and 202, either or both of which can besteering mirrors. These steering can be used to compensate forwavelength dependent variations in the beam angle of the IR source 112,can be used to compensate for other deviations of the IR beam, forexample due to temperature or other system variations. One or moresteering mirrors 200 and 202 can also be used to align the IR beam tothe sensing or probe beam 127 and/or align the IR heating beam relativeto focusing objective 112, also discussed below. Beam steering mirrorsmay be fast steering mirrors, for example actuated by voice coils,electrostatic drives, and/or piezo elements, for example, or may besingle or multi-axis galvo mirrors, MEMS-based mirrors or other devicesthat steer a beam in response to an external control signal. Afterpassing the optional steering mirrors, the IR heating beam may bedirected towards beam combiner 116 and focused on the sample asdescribed above for FIG. 1B.

Turning now to the sensing or probe beam, a probe beam source 126 emitsa probe light beam 127 as in FIG. 1B. Optional beam steering mirrors 204and 206 can be used in a way analogous to beam steering mirrors 200 and202 described previously. In addition these mirror can be used to adjustthe position of the probe beam focused spot relative to the IR beamfocused spot, as described associated with FIG. 5 . This feature can beused to accomplish three separate tasks: (1) centering the probe beam atthe point of maximum intensity of the IR spot to maximize thesensitivity of the measurement of IR absorption; (2) scanning the probebeam over a plurality of position in the probe beam to measure the IRabsorption response at a plurality of position on the sample; (3)controlling the degree of overlap of the IR and probe beams for exampleto improve the spatial resolution of the measurement beyond even thediffraction limit of the probe beam. Each of these will be discussed inmore detail associated with FIG. 4 .

The probe beam optical path can also include additional optional opticalelements for example half wave plate 212 and quarter waveplate 214 toincrease the efficiency of illumination of the sample with the probebeam and detection of the deviation in the probe beam reflected from thesample. This can be achieved by the use of a polarizing beamsplitter forbeamsplitter element 128. The half wave plate 212 can be used to rotatethe probe beam polarization to maximize the transmission throughpolarizing beamsplitter 128. Quarter waveplate 214 can be used to rotatethe polarization of the reflected probe beam by 90 degrees relative tothe incident probe beam. This allows the beamsplitter 128 to selectivelyreflect the returning probe beam towards received 142 with minimaloptical losses.

The apparatus may also incorporate an illumination system and camera 210to visualize the sample and if desired one or more of the optical spotsfrom the IR and/or probe beam. In this embodiment, mirror 118 of FIG. 1Bis replaced with either a removable mirror, rotatable mirror, and/ordichroic mirror. In this embodiment, light from the illuminator/cameramodule can pass through focusing optic 120 to illuminate sample 125 andscattered/reflected light can be collected by optic 120 and returned tothe illuminator camera module 210 to produce optical images of thesample. Mirror 208 can be under computer control to automatically switchin and out as needed. Alternately the sample can be illuminated intransmission from below via optic 134 or the sample can be illuminatedabove and the camera can be placed under the sample. The camera 210 canalso be used to record laser speckle resulting from probe lightscattered from the sample. This speckle pattern can be especially usefulfor recording the photo-thermal response of rough samples under IRabsorption, for example as described by Sullenberger et al (DOI:10.1364/OL.42.000203), which is hereby incorporated by reference. Aspeckle pattern is the result of the interference of light reflectedfrom many small asperities of a rough sample with random phases. As thesample heats up due to IR absorption changing the local index ofrefraction and/or free surface shape, the speckle pattern can readilychange due to optical phase changes from the index of refraction and orsurface deformation. The camera for example can be used to identifypixels where the speckle intensity changes as a function the incident IRradiation. For example, it is possible to bin the signal from camerapixels that have a high speckle intensity and calculate the variation inintensity and/or position as a rate synchronized to themodulation/pulsing of the IR source.

FIG. 3 is a simplified schematic diagram of an embodiment of the currentapparatus showing further optional features. FIG. 3 has many commonfeatures with FIG. 1B and FIG. 2 . As such the same label numbers areused in FIG. 3 for common features and the associated descriptions applyas appropriate. In FIG. 3 , receiver 142 from FIG. 1B is shown in moredetail comprising elements (including elements 302 to 312) to bothimprove the sensitivity and to perform multiple simultaneous imaging andspectroscopic modes, including for example IR spectroscopy, fluorescenceimaging, and/or Raman microscopy/spectroscopy. For simplicity, FIG. 3shows only a reflection configuration, but the following discussion canapply equivalently for a receiver module position to collectedtransmitted probe radiation, for example element 140 in FIG. 1B and FIG.2 .

As with FIG. 2 , light reflected/back scattered from the sample iscollected by optic 120, returned along the incident beam path anddirected via beamsplitter 128 to the receiver module 142. Receivermodule 142 can comprise multiple detectors, mirrors, spectrometers,filters, etc. as needed to direct the probe beam 129 to one of moresensor units. For example, a flipper mirror, rotatable mirror, removablemirror, or dichroic mirror 302 can direct probe light to a focusing lens314 where the light can be focused into an optical fiber coupler 315.From the fiber coupler 315 the light can be directed via optical fiber316 to any external/remote sensor or spectrometer 318, which cancomprise for example a Raman spectrometer. The Raman spectrometer canalso be coupled by a free space beam and can be located entirely withinor directly attached to receiver 142. If mirror 302 is rotated orotherwise removed from the beam path, probe light can pass onto othersensors, detectors, and/or spectrometers. For example, mirror 304 canreflect light to an optical detector 312 to be used for fluorescencemeasurements. Mirror 304 can be a rotatable/removable mirror asdescribed for mirror 302 or it can be a dichroic mirror selected toreflect light of the fluorescent wavelength of interest. Optional filter310 can also or instead be used to select the fluorescent wavelength.Again, both optical paths are shown, but any combination of theseadditional paths may be employed.

For measurements of IR absorption, the collected probe light can also bedirected to a detector 308 that is selected to be highly sensitive tothe probe light wavelength. In this embodiment, the detector signal maybe amplified and routed to analyzing electronics 144 as described withrespect to FIG. 1B and FIG. 2 to create measurements of IR absorptionvia detection of modulation of the probe beams angle, size and/orintensity.

To increase the sensitivity, it may be desirable to include a beamshaping element 306. This element may be a manual or motorized iris forexample that clips a portion of the collected probe beam. In thisembodiment, as the angular distribution of the probe beam changes inresponse to the IR absorption of the sample, the angular distributionchange will result in an amplitude modulation of the beam. The beamshaping element can alternately or additionally include a centralobscuration that blocks the center part of the beam. The reason for thisis that the photo-thermal distortion of the probe beam can affect theextreme rays the most. So blocking the central rays with an obscurationcan block light from hitting the detector that does not contributesignificantly to the demodulation signal. Blocking central rays orotherwise selecting a smaller portion of the probe beam the detector canallow the detector amplifier electronics to be operated at higher gainand/or longer integration times before saturating. Alternately there canbe an additional lens in the collection arm that focuses beam 129 to aspot either at or before the probe light detector 308.

In one embodiment, the additional lens focuses the beam to a post beforethe detector, and a pinhole may be placed at the focused spot to blocklight that is scattered or reflected from regions outside the samplefocal plane. Probe light detector 308 can be a detector that measuresthe relative intensity of the beam incident on it, for example aconventional photodiode, an avalanche photodiode, photomultiplier tube,and/or other detector that produces a signal that a signal indicative ofan intensity of the light incident on the detector. Alternately, thedetector 308 can be a position sensitive detector, for example a linearphotodiode, a dual or quad segment detector or a multi-detector array.In this embodiment, the detector can also be sensitive to positionalshifts in the reflected/scattered beam, for example due to angulardeviations in the beam and/or lateral shifts. Alternately detector 308may comprise a phase sensitive detector, comprising further aninterferometric detection scheme that produces a signal indicative ofthe optical phase or optical phase shift of the beam incident on thedetector. In these embodiments, the system can measure the change inintensity, beam angle and/or optical retardation induced by atemperature change in the sample due to the interaction or absorption ofinfrared light by the sample. Detector 308 may also be an array detectorand/or a camera that is sensitive to light at the probe wavelength. Inthis embodiment, the array detector/camera may be used to track thedeviations in intensity and/or position of light reflected or scatteredfrom the sample, including the analysis of speckle as described in thedescription of FIG. 2 .

The receiver can comprise a camera and/or an array detector. The use ofa camera and/or array detector can provide a substantial improvement inmeasurement throughput by enabling parallel measurements of multiplelocations on the sample simultaneously. To achieve high spatialresolution, it may be desirable to use a camera or array detector thathas a fast response time, or equivalently a high measurement bandwidth.A reason for this is that as IR light or another heating beam isabsorbed by the sample, the absorbed heat can diffuse away from theabsorbing region, causing a reduction in the spatial resolution. Tomaintain high spatial resolution, it can be desirable to employ IR lasersources with high repetition rates and detectors with high bandwidthsthat enable measurements off the probe light on timescales shorter thanthe thermal diffusion time for a desired spatial resolution.

The thermal diffusion time constant τ is given by the equation:τ=2πμ²/αwhere μ is the thermal diffusion length and a is the thermaldiffusivity. One limit on spatial resolution is set by the thermaldiffusion length μ. To achieve a specific spatial resolution, it isdesirable to keep the thermal diffusion length μ smaller than the targetspatial resolution. Then, the probe light responses are read ontimescales that are shorter than the thermal diffusion time constant τ.So, for example, using a thermal diffusivity of 1.35×10⁻⁷ m²/sec (atypical value for polymeric materials), and a desired spatial resolutionof 500 nm, the thermal diffusion time constant τ is 1.17×10⁻⁵ sec, andfor 200 nm, τ is 1.87×10⁻⁶ sec. In certain embodiments, probe light issampled on time scales at these times or shorter to achieve spatialresolutions comparable to the spatial resolution achievable with thefocused spot size of the probe beam.

In some embodiments, it may be desirable to use cameras and/or arraydetectors that are capable of fast response/readout time. A few of thespecific desirable characteristics for such cameras/array detectors are:

-   a) Fast Response times on the order of high repetition rate lasers.    For instance, in one embodiment, the IR laser used has a repetition    rate of up to 1 MHz and these rates could go up in the future.-   b) Can respond to a trigger signal which leads to a gate length of    camera exposure that can be on for time durations less than the    thermal diffusion time constants mentioned above.-   c) Can collect a separate background signal (either before the    experiment or in real time between each signal collection pulse) and    mathematically process the signal and background signals to create a    background corrected signal.-   d) Can co-add the signals corresponding to each laser pulse to    achieve the desired Signal level for the experiment.

There are a number of cameras and detectors that can be suitable forhigh bandwidth detection. Researchers at the Non-EquilibriumThermodynamics Laboratory at Ohio State University and theircollaborators have demonstrated MHz frame rate camera detection withhigh repetition rate laser sources. (See for example N. Jiang, W.Lempert, G. Switzer, T. R Meyer and J. R. Gord, “Narrow-LinewidthMegahertz-Repetition-Rate Optical Parametric Oscillator for High-SpeedFlow and Combustion Diagnostics”, Applied Optics, vol. 47, No. 1, pp.64-71, 2008.) Princeton Instruments, for example makes high speed, highsensitivity cameras for spectroscopy and 1magmg. The Pixis cameras canoperate at sampling speeds of 100 kHz and 2 MHz in a variety of pixelconfigurations. Princeton Instruments' PiMax4 cameras are optimized forhigh repetition rate laser spectroscopy and can operate at repetitionrates of up to 1 MHz and can use a trigger with a gate length adjustablefrom 500 ps onwards. These cameras can collect signals in 2 separatebuffers that can each be triggered separately and co-added separately(or mathematically processed relative to each other via a subtraction ordivision or other mathematical function). Teledyne Dalsa makes a linearray camera with 100 kHz/200 kHz line rates. Horiba similarly makes CCDarray cameras readout rates in the range from −10 kHz to 3 MHz. It isalso possible to use linear arrays of UV/visible light detectors.Hamamatsu, OSI, and other vendors make linear arrays for example in 16,46, and 76 elements. These detectors have high optical responsivityenabling sensitive detection and have low capacitance for high speedreadout in the range from 10 s of kHz to MHz or higher.

In either embodiment, light signals from the array/camera elements canbe digitized and analyzed. In one embodiment, the probe light signalsfrom the array/camera elements can be demodulated at a frequencycorresponding to the laser repetition rate or a harmonic thereof. Thisdemodulation can be performed by a lock-in amplifier, a series ofparallel lock-in amplifiers, and/or their digital equivalent. Forexample, the demodulation can be performed by computation in a computer,digital signal processor, field programmable gate array, or anycombination thereof or other suitable digital computation means. It canalso be desirable to perform time-domain analysis, for example comparingthe signal strength at a time window after the start of the IR laserpulse to the signal strength during a time where the laser pulse is offFor high spatial resolution, it can be desirable to sample thecamera/array detector on timescales similar to, or shorter than, thethermal diffusion time described above. In some embodiments, this mayinvolve examining the probe light response at early times after the IRlaser pulse and before thermal diffusion has had the opportunity todegrade the spatial resolution.

With an appropriate high acquisition speed camera, a variety oftechniques may be accomplished with a photo-thermal imaging system.Using optics of the appropriate magnification a desired region of thesample at the probe laser wavelength may be imaged onto a 2D pixel arraya camera. Using a pulsed tunable IR laser a desired sample region, maybeilluminated, possibly at a number of different of illuminationwavelengths, one wavelength at a time. A broadband source could also beused.

A sensing beam (also referred to as a probe beam) can be produced by aprobe light source that could be any of a variety of devices, includinglaser or LED sources, in combination with the source of the heatingbeam. The reflected light from the source of the probe beam is imagedonto the camera sensor at a desired magnification. The probeillumination can be photo-thermally modulated by the absorbance of theheating beam by the sample.

A first trigger signal can be created that is coincident with the IRlaser pulse and has a gate that allows camera exposure for a timeduration less than the thermal diffusion time constant of the samplebeing studied. The appropriate delay from the start of the laser pulseto achieve the maximum photo-thermally modulated probe signal on thecamera can be determined. This trigger signal (with the appropriatedelay) can trigger capture of at least one frame of image data.

A second trigger signal can be created that captures the probe beam thatis reflected from the sample in the absence of the heating beam. Thiscorresponds to a background signal and can be captured as second set ofcorrection frames. The trigger signals can have the same gate time butwill be timed so that the second trigger occurs during a time of noillumination by the heating beam.

The two sets of frames can be mathematically processed to obtain abackground corrected signal. Processing could be a subtraction; divisionor other mathematical function. The frames processed the backgroundcorrected photo-thermal signal may be the actual data frames andcorrection frames can be applied as many times as needed to get anexperimentally desired signal frame. The number of applications of thecorrection data can be determined experimentally. For instance,increasing the power of the heating beam source and/or the power of thesensing beam source as needed can increase the signal and minimize thenumber of corrections needed.

In an embodiment in which a source of broadband IR illumination is used,then a Fourier transform of the data will be taken at an appropriateplace in the process, similar to the process used in a conventional FTIRmicroscope with a camera. Sub-diffraction spectroscopy and imaging orany other long wavelength spectroscopy and imaging can be accomplishedfor IR heating beams or even for other, longer wavelengths.

Because the gate following the trigger signal on a suitable camera canbe made as narrow as 500 ps or 0.5 ns, then time resolved spectroscopymay be performed by measuring the spectra at different time scales withthe camera.

In photoacoustic spectroscopy, by varying the modulation frequency (orrepetition rate) of the incident heating beam illumination, informationcan be obtained from different depths of the sample, with higherfrequencies (or higher repetition rates) giving information fromshallower regions and lower frequencies (or lower repetition rates)giving spectra information from deeper in the sample. So, by varying therepetition rate in discrete intervals of 1 KHz, for example, whenobtaining the IR spectroscopy and imaging information, subtracting theinformation from the different repetition rates may be used to obtain IRspectroscopy and imaging information from different slices at differentdepths. Depth profiling information could also be obtained by comparedthe delay times that produce a maximum absorbance signal at eachwavenumber, as described above.

Signal frames may be acquired using background compensation at multipledelay times after triggering. At the end of the full spectrum datacollection at each delay time, determine the delay time at eachwavelength which produced the maximum difference signal from thebackground may be determined. Those wavelengths with longer delay timesfor maximum difference peak absorbance signal likely originated fromdeeper in the sample, as there would be a time delay for thephoto-thermal response to reach the sample surface.

By studying the variation of the photo-thermal signal intensity acrossthe camera pixel array, the angular dependence may be analyzed of thisphoto-thermal signal for different heating beam wavelengths and thisinformation may be used to improve the technique sensitivity, for usewith a 1 dimension line array detector or also with a single pointdetector. Because the visible probe laser may be continuous wave, therewill usually be a significant background signal even when the pumped IRpulsed source is not illuminating the sample. Use of a large-format,fast acquisition visible camera, will also allow probing of the angulardistribution of reflected light containing the modulated IR absorptioninformation from the illuminated area of the sample. In someembodiments, the maximum background compensated signal will occur atspecific pixel locations (resulting from different reflection angles).By selecting only these hot spots in binning the pixels, higher signalto noise ratios can be achieved. By adjusting the incidence angle of thevisible probe beam, it may be possible to optimize the difference signalamplitude between the unmodulated background and the modulated signalwhich arises from absorption of the heating beam by the sample.

Optical properties of larger areas of the sample can be measured with asingle snapshot. By illuminating a larger area of the sample with boththe heating beam and the sensing beam, it is possible to obtain IRtransmission-like spectra in a reflection configuration using a fastacquisition camera system. The measurements in this mode may no longerbe at sub-diffraction-limited spatial resolutions, but larger areas ofthe sample can be examined in a single measurement. In some embodiments,all of the pixels in the area could be combined and normalized toproduce a single average IR spectrum of an illuminated area of thesample.

A continuous wave source of probe illumination could be set up passadjacent to the sample surface. The IR pulsed beam would illuminate thesample at much smaller angles of incidence relative to the samplenormal. The gas phase molecules in the path of the probe beam willproduce a modulated effect when the wavelength of the pump beam isabsorbed by the sample. The probe beam modulation could then be detectedas a function of angle using either a one-dimensional or two-dimensionalfast acquisition visible array camera.

Mirrors 302 and 304 can alternately be beamsplitters that divide thelight between multiple sensors allowing simultaneous measurements of IRabsorption, Raman scattering, and/or fluorescence intensity. Thebeamsplitters can also include dichroic coatings to separate the probelight by wavelength. Fluorescently scattered and Raman scattered lightwill return at a different wavelength than the probe light excitationwavelength. Since IR absorption information is carried by light at awavelength similar to that of the sensing beam, separating out aminority of light that is scattered at fluorescent or Raman wavelengthshas a minimal impact on the PTP sensitivity. As such it is possible tosimultaneously measure IR absorption, Raman, and/or fluorescence of thesame region of the sample at the same time, and with the same probeexcitation beam. In this embodiment, “simultaneously” is meant to conveythat both the IR and Raman measurements are performed at substantiallythe same time, not sequentially. However, the data collection may beperformed sequentially as well, even if the collection elements operatein parallel. Specifically, in this embodiment the system does not needto be reconfigured between measurements and that the measurement of theIR response does not block the measurement of the Raman response or viceversa. This feature has an advantage in terms of measurement throughput.For example, if a series of IR and Raman measurements would each take 30minutes over some specified area in a conventional system, the abilityto perform the measurements simultaneously reduces the total test timeto 30 minutes, rather than 60 minutes. “Simultaneously” as used in thisembodiment does not mean that the IR and Raman data is necessarilysampled at exactly the same microsecond, but instead that the twomeasurements can be done substantially in parallel.

FIGS. 4A-4I show simplified conceptual diagrams illustrating therelative size and overlap between the IR beam and probe beams. In manyembodiments, the probe beam will have a shorter wavelength than the IRbeam such that it can be focused to a smaller diffraction limited spotthan the IR beam. In general, the focused spot of the probe beam willilluminate a region of the sample that comprises smaller subset of thefocused IR spot, as illustrated in FIG. 1A and in FIG. 4A. In FIG. 4A,IR beam 400 is focused onto a sample 402. The IR beam comes to a beamwaist 404, the narrowest part of the focused beam, at or near a plane ofinterest in the sample. In this embodiment, it is not necessary thatthis plane correspond to a surface of the sample; the beam waist mayinstead be internal to the sample.

An example of the overlap of the IR and probe beams are shown incross-section if FIG. 4B when the IR and probe beams are substantiallycentered. FIG. 4C shows a cross-section through the intensity profilesof the two beams as overlapped in FIGS. 4A and 4B. The intensity of thedeviation of the probe beam generally occurs when the intensity peaks ofthe probe beam and the intensity of the IR beam are substantiallyoverlapped. Beam steering mirrors, for example 200, 202, 204, and 206 inFIGS. 2 and 3 can be used in any combination to achieve the optimalalignment between the IR beam and the probe beam to maximize thesensitivity of the probe beam to IR absorption. This can be doneautomatically by sweeping the position of one or more of the beams whilemeasuring the demodulation signal and selecting the positions of thesteering mirror(s) that substantially maximize the signal strength.

It is also possible to sweep the position of the probe beam 408 relativeto the IR beam 400. As indicated in FIG. 4D the focused spot 404 of theprobe beam can be swept over a plurality of locations within IR beamwaist 404 to map probe beam deviation and hence IR absorption at aplurality of positions on the sample. In this embodiment, the intensityof the IR beam will vary over the focused spot area, so it can bedesirable to normalize the measured probe response by the intensity ofthe IR beam at a given location of the probe beam. FIGS. 4E and 4F showanalogous figures to FIGS. 4B and 4C except with the probe beam focusedat a plurality of positions, not necessary simply centered with the IRbeam. FIG. 4G shows an alternate positioning of the probe beam withrespect to the visible beam that can provide spatial resolution evenfurther below that of the diffraction limit of the probe beam. In thisembodiment, the probe beam is positioned at a point where the focus spot404 of the probe beam is not entirely enclosed in the beam waist 404 ofthe IR beam. As shown in the cross-section image FIG. 4H, this leads toa situation where there is only a partial overlap 414 of the two beams.As there is only a signal at the demodulator when there is bothsimultaneous absorption of IR by the sample and readout of theabsorption by the probe beam, the intentional mis-alignment of these twobeams can reduce the spatial resolution to the size of the overlap, notset by the size of the probe beam. In an example discussed earlier, a405 nm laser can be focused to achieve a diffraction limited resolutionof around 260 nm. If this spot is aligned as shown schematically inFIGS. 4G-4I, it is possible to arrange an overlap much smaller than the260 nm Abbe diffraction limit.

With this approach, it is possible to achieve a spatial resolution ofless than 100 nm. Normal Gaussian beams will have a significant drop-offin intensity away from the point of maximal overlap, but this can betolerable in embodiments where higher spatial resolution is required andsensitivity can be sacrificed. Alternately it is possible to employoptics to shape one or both of the beams into so called “flat-top” beamsinstead of Gaussian profiles. Even in the embodiment that the beamshaping optics result in larger focused beams, the spatial resolutioncan still be improved below the conventional diffraction limit bycontrolling the amount of beam overlap.

Spatial resolution. The spatial resolution that can be achieved can beset by one or more of (1) the size of the probe beam; (2) the area ofoverlap of the IR and visible beams; (3) the frequency of modulation ofthe IR beam. When the modulation frequency of the IR beam is high enoughsuch that the thermal diffusion length is much smaller than the spotsize of the probe beam, then the spatial resolution can be much smallerthan the IR beam spot size. In various embodiments, it is possible toachieve spatial resolution of less than 1000 nm, less than 500 nm, andless than 100 nm.

FIG. 5 shows a simplified schematic diagram of an embodiment amicroscopy and analysis platform employing the photo-thermal detectionsystem described previously, but in this embodiment operating in avacuum environment. The ability to perform this measurement in vacuumenables the ability to perform analyses by infrared spectroscopy, Ramanspectroscopy, and/or fluorescence imaging along with collocatedmeasurements by any number of vacuum analytical techniques, including,but not limited to: scanning electron microscopy (SEM), transmissionelectron microscopy (TEM), x-ray photoelectron spectroscopy (XPS), x-raydiffraction (XRD), energy dispersive X-ray spectroscopy (EDS), time offlight mass spectrometry (TOF-SIMS), mass spectrometry (MS), and atomicforce microscope-based mass spectrometry (AFM-MS), for example. FIG. 5shares many components with previous diagrams and similar elements sharethe same numerical labels as previous drawings and associateddescriptions apply as appropriate. As before, both heating beam source112 and a sensing beam source 126 direct a corresponding heating beam114 and sensing beam 130 on a parallel and overlapping path via optionalsteering mirrors (not shown in this embodiment) and beam combiner 116.The combined beams may be reflected off additional steering and beamconditioning optics, for example turning mirrors 500, 502, and 504before impinging on a window 506 in a vacuum chamber 508. The window isselected to be transmissive to both the IR and probe beams over thewavelength range of interest. Note that it may be desirable to have theprobe beam parallel to but not collinear with the pump beam so that itcan pass through an optional hole in any antireflection coating in thewindow that is optimized for the IR radiation. Once in the vacuumchamber the combined beam may be focused by focusing optic 120,analogous to the focusing optics shown in FIGS. 1B-3 . In the embodimentshown focusing optic 120 is inclined relative to the sample to provideaccess for other analyzers, shown schematically by blocks 518. Theanalyzer 518 may comprise one or more electron guns, ion beams, ionfocusing optics, X-ray sources and optics, mass analyzers, chargedetectors, sensors, etc. in support of the vacuum analysis techniqueslisted earlier.

After the combined IR and probe beams interact with region of interest124 of sample 125, the reflected/scattered beam 512 can be collected bycollection optic 134 for analysis. The desired detectors can be placedinside the vacuum chamber and/or the light may be coupled out throughanother window 513, which can then be coupled to one or more mirrors orother beam steering/beam conditioning optics, for example mirrors 514and 516, before entering receiver 142. As described previously receivermodule 142 can comprise one or more detectors and spectrometers toanalyze the collected light to perform IR spectroscopy, fluorescence,and/or Raman via analysis of the probe beam returning from the sample.The sample 125 can be mounted on one or more translation/scanning stages119 that can optionally be mounted on mount 511 providing rigid orvibration isolating connection to a surface of the vacuum chamber. Thetranslation/scanning stage 119 allows coarse positioning of the sampleand/or performing measurements at a plurality of locations on the sampleto create a plurality of spectra 149 and/or chemical/compositional maps150. This substantially simplifies the required optical path andmechanical access required to make multiple analytical measurements,enabling compatibility with other vacuum analytical techniques.

In FIG. 5 the illumination and collection optics 120 and 134 are showninclined relative to the sample to provide access in the vertical planefor one or more of the analytical techniques described above, whilebeing able to illuminate and collect light from the sample in areflection/forward scatter configuration. In alternative embodiments, itis possible to operate in a transmission configuration, for exampleplacing the collection optic 134 underneath the sample. The illuminationfocus optic 120 and collection optic 134 can be inclined as in FIG. 5 orif desired can be positioned on a vertical axis similar to that shown inFIG. 1B.

FIG. 6 shows an alternative embodiment for operating the current devicein a vacuum chamber. FIG. 6 is very similar to the apparatus shown inFIG. 3 and the associated descriptions apply to like-numbered parts. InFIG. 6 , in contrast to FIG. 3 , a portion of the apparatus is enclosedin vacuum chamber 508 similar to the vacuum chamber having the samereference number described above with respect to FIG. 5 . Vacuum chamber508 can be evacuated to be compatible with various vacuum analyticaltechniques. After heating and sensing beams are combined at beamcombiner 116, the combined beam passes through window 506. Once invacuum the combined beams can reflect off optional mirror 208 and arethen directed into focusing optic 120 that focuses the combined beams122 to spots on a region of interest 124 of sample 125. In theembodiment shown the focusing optic 120 is oriented substantiallyperpendicular to the sample to provide access for beams and detectors ofvacuum analytical techniques in complementary solid angles. As describedearlier mirror 208 may be a dichroic mirror to provide an optical pathto a camera 210 that can be placed either inside the vacuum chamber oras shown outside the vacuum chamber, positioned outside a window 600.

FIG. 7 shows an alternative embodiment of the current apparatus forperforming IR spectroscopy, Raman spectroscopy, and other analyticaltechniques, within vacuum. The embodiment shown in FIG. 7 is a variationof the simplified schematic of FIG. 1B, but the concepts described belowcan be applied in any combination with various other vacuum andnon-vacuum embodiments. In various embodiments, the sample 125 is placedon a long travel stage 700 that can shuttle the sample between twodifferent location to position the sample at two or more locations 702and 704 that are centered under different analytical techniques.

In the embodiment shown, sample location 702 positions the region ofinterest 124 under the focused IR and probe beams 122. At this location,it is possible to perform IR spectroscopy, Raman spectroscopy and/orfluorescence measurements. In the second sample location 704, the sample125 is shuttled such that the region of interest 124 is centered underone or more vacuum analytical techniques, for example SEM, TEM, XRD,TOF-SIMS, EDS or other similar techniques. The objects labeled 518represent for example electron guns, ion guns, mass samplers, ionoptics, X-ray sources, collectors, detectors, etc. as needed to performthe desired analytical technique. This shuttle arrangement is desirablein a situation where the required solid angle required by the vacuumanalytical techniques does not provide sufficient physical and/oroptical clearance for the focusing/collection optic 120 and theassociated illumination and sample return beams 122. The objects 518 canalso or alternatively be a device for etching the sample, for examplevia focused ion beam etching, plasma etching or other etching methods.In this embodiment, the system can be used to perform chemicaltomography by alternately etching and analyzing the sample. For example,the sample can be positioned at location 702 and have the surfaceanalyzed by IR spectroscopy, Raman spectroscopy, and/or fluorescence,and then shuttled to position 704 to remove a desired amount ofmaterial, then returned to location 702 for subsequent chemicalanalysis. By repeating this process, it is possible to reconstruct athree dimensional map of the chemical composition of the sample. Vacuumetch technique exist that can be used to remove nanometer level amountsof material. So, it is possible to create a 3D map of the samplecomposition with nm vertical spatial resolution and lateral spatialresolution below 1000 nm and even below 100 nm using the partial beamoverlap scheme described earlier.

Another advantage of the photo-thermal IR technique is that it overcomesthe limitations of conventional reflection based IR spectroscopy. Inconventional reflection-based IR spectroscopy, the sample is illuminatedwith IR light the light that is transmitted through the sample orreflected/scattered from the sample is collected and analyzed.Transmission IR absorption spectra are extremely useful for analyzingthe chemical makeup of a sample because there are vast databases withbulk transmission spectra for hundreds of thousands of materials.Transmission IR spectroscopy, however, generally requires preparing athin section of a sample, which can range from time consuming toimpractical or impossible, depending on the type of sample.

Alternative IR spectroscopy techniques exist that are based onreflectance arrangements, where the system can measure the IR light thatis reflected and/or scattered from the sample. This can eliminate theneed to prepare a thinly sectioned sample, but reflection-based IRspectroscopy has many artifacts relative to transmission IRspectroscopy. The artifacts are primarily due to a variety of factorsresulting from the scattering of light and dispersive effects resultingfrom contributions of both the real and imaginary components of theindex of refraction in the amount of reflected/scattered IR light. Theseartifacts frequently distort measured IR spectra versus transmission IRspectra, making chemical analysis and identification significantly morechallenging.

An alternative technique is attenuated total reflection (ATR). ATRbrings an IR transparent crystal into contact with a sample and thenilluminates the crystal and sample with IR light in total internalreflection configuration. The ATR approach also eliminates the need forthin sections, although the spectra can still be distorted relative totransmission spectra. Further, the required contact between the ATRcrystal and the sample can be difficult to accomplish for some samplesand measurements, and can especially be challenging for imagingapplication.

The various embodiments described herein overcome the limitations oftransmission, reflection and ATR-based IR spectroscopy. Using adual-beam detection beam having a sensing beam and a heating beam, it ispossible to avoid the reflection artifacts and collect undistorted IRabsorption spectra that are a good match to conventional transmission IRspectroscopy, thus enabling the use of materials databases tocharacterize and/or identify an unknown material. The methods disclosedherein improve both detection of the chemical composition of the sample,and also improve upon the imaging resolution compared to conventionalsystems that were limited by the diffraction limit of an IR beam (orother long wavelength heating beam like terahertz radiation). FIG. 8shows a simplified schematic diagram for an embodiment where the currenttechnique is used to obtain high quality transmission-like IR absorptionspectra over various length scales, ranging from <100 nm to many mm.FIG. 8 is similar to FIG. 1B and where common callouts are used, thediscussion associated with FIG. 1B applies. As with FIG. 1B, in FIG. 8infrared beam 114 and probe beam 127 are combined with beam combiner 116and directed towards an optic 800. Unlike FIG. 1B, however optic 800 isnot necessarily a focusing optic that focuses the IR and probe beams tosmall focus spots. Instead, optic 800 may in fact expand the beam 802 tocover a wide area of the sample 125.

Alternately optic 800 may be a focusing optic like focusing optic 120 ofFIG. 1B, but positioned at a distance from the sample where the sampleis out of focus such that the illuminated area is larger than at thefocus. In either embodiment, probe light reflected and/or scattered fromthe wide area of the sample may be collected by the same optic 800 or byan additional optic (not shown) positioned at an angle relative to optic800 and the sample 125. In either embodiment, probe light that iscollected after interacting with the sample can be directed to either areceiver 142 as described with FIG. 1B and following figures and/or canbe directed towards a camera 210. The camera can capture the probe lightthat is reflected or scattered from a wide area of the sample. Thecamera can be synchronized to the pulse control sync signal 146 that isused to pulse/modulate the IR source. The camera can be programmed toacquire one or more frames when the IR laser pulse is on and one or moreframes when it is off, in embodiments. By comparing the differentialsignals with the heating beam either on or off, over an accumulatednumber of frames it is possible to determine the change inreflected/scattered intensity at each of the pixels in the field of viewof the camera.

Alternately, the camera can be used to acquire a time series of data fora plurality of pixels and then a computational device (controller) canbe used to demodulate the time series of data at a frequencycorresponding to the pulse control sync signal 146, typically eitherdirectly at the modulation frequency or an integer harmonic thereof. Forexample, an FFT can be performed on the pixel time series data, or adigital lock-in algorithm or any similar technique to demodulate thepixel time series data into a signal indicative of a modulation in thesignal induced from the IR absorption by the sample. Via this approach,it is possible to obtain signals indicative of IR absorption spectrathat are spatially resolved via the camera detection and/or averagedover a large area, for example by sending light collected over a widearea to the receiver 142.

It is also possible to change the widefield optic 800 to a collimatingoptic (for standoff measurements of distance objects) or to a focusingoptic 120 for microscopic measurements, to then provide focused IR andprobe beams 804 to perform a measurement on a small area. In thisembodiment, the wide field configuration using wide field beams 802 canperform a large area survey measurement and a microscopic measurementcan be performed with focused beams 804. In the embodiment of usingfocus beams, it is possible then to achieve a spatial resolution of IRabsorption of 700 nm or less by focusing the probe beam using high NAoptics as described previously. As mentioned previously, it is alsopossible to conduct atomic force microscopy based IR spectroscopy(AFM-IR) in parallel with the large survey measurement to achieve IRabsorption measurements, down to the scale of −10 nm. Depending on thedesired data, this can be accomplished by either by swapping optics,changing the internal configuration of the optic and/or changing thevertical position of the optic 120 or 800 relative to the sample 125. Inthe embodiment of wide field illumination for example using the widefield optic 800 of FIG. 8 , it may be desirable to use high power IR andvisible laser sources to maintain similar optical power density as usedfor microscopy applications. For microscopy applications where the beamsare focused to diffraction limited spots, it is common to use beams withaverage most often in the scale of about 1 mW to about 10 mW, althoughfor some applications either higher or lower power can be used.Available tunable sources in the mid-IR can produce output beams on thescale of several hundred milliwatts to many watts of optical power. Forexample, the Firefly-IR from MSquared Lasers can produce >250 mW ofmid-IR radiation. Nanosecond optical parametric oscillators (OPO) forexample from Ekspla can produce 450 μJ at 1 kHz or 450 mW of IR power.Amplitude Systems produce an OPO laser that can take up to 50 W of inputpower and produce mid-IR output with >12% efficiency, producing forexample 6 W of mid-IR energy. Jiang et all have demonstrated high powerpulsed OPOs with up to 8.5 W of power at 3.3 um wavelength(DOI:10.1364/OE.23.002633). Peng et al. demonstrated a tunable mid-IRlaser with output power in excess of 27 W(DOI:10.1134/S1054660X1201015X). Hemming et al. demonstrated a highpower mid-IR ZGP ring OPO with over 30 W of optical power (DOI:10.1364/CLEO_SI.2013.CWlB.7)

Each of these mid IR sources would in turn provide sufficient power toilluminate increasingly larger areas of the sample with the same averagepower density as used for the microscopic focus embodiment. For example,assume that for the microscope embodiment the IR beam is focused to a 10μm spot with 1 mW of power for sufficient sensitivity. With 30W of IRpower from the Jiang et al OPO, it is possible to illuminate an areawith a diameter 122× bigger than with a 10 μm focused spot (via thesquare root of the power ratio of 30 W to 1 mw). So, this wouldcorrespond to the ability to illuminate an area 1220 μm in diameter.Similarly, visible, UV and near IR pump lasers are available withextremely high power as well. For example, 532 nm green laser systemsare available from Optotronics with up to 6 W of power. Coherentmanufactures a 1064 nm laser with up to SSW of optical power. Thus, thisembodiment could provide measurements over a sample area of more than 1mm in diameter, or areas greater than 1 square mm. Visible, near IR, UVand mid-IR laser power sources are increasing in power output over time,so the sample area available for IR spectroscopic measurement, as wellas collated Raman and/or fluorescence, can be expected to increase asmore powerful sources become available.

The dual beam technique described above produces spectra that aresubstantially free of dispersive artifacts associated with conventionalreflection/back scattered IR spectroscopy. Because the probe beam issensitive only to the change in temperature of the sample due to thephoto-thermal response (e.g., from change in index of refraction and/orthermal expansion), it is completely independent of the wavelengthdependent reflectivity and/or scatter of the IR beam. As such thedisclosed techniques provide the ability to provide IR absorptionspectra that correlate well to conventional FT-IR spectroscopy,substantially without dispersive artifacts, and allow accurate materialcharacterization and identification. These characterization spectra caninclude in mid-IR wavelengths where the so-called “fingerprint bands”exist that provide rich information for discriminating materials, evenhighly similar materials.

It is not strictly necessary for the sensing or probe beam to have ashorter wavelength than the heating beam, in embodiments. While this isdesirable for microscopic measurements where the desired spatialresolution is smaller than the optical diffraction limit of the heatingbeam, it is not needed for wide area measurements with coarser spatialresolution requirements or for bulk measurements. Rather, it can besufficient in some embodiments to use a substantially fixed wavelengthprobe beam, rather than necessarily a shorter wavelength. Using a fixedwavelength probe beam allows the measurement of the photo-thermaldistortion of the sample with no issues of wavelength dependentvariations in the optical properties of the sample as is the embodimentwhen measuring an IR response from the heating beam by observing thereflection over the IR wavelengths of the incident beam. This insightallows the application of photo-thermal IR technique using probe beamsthat are of any desired wavelength that produces sufficiently high powerto produce a detectable signal.

Probe or sensing beams can be produced by sensing beam sources such aslasers operating in the UV spectrum, visible spectrum, near IR spectrum,or even the mid-IR spectrum. It also may not be necessary to employ thesame power density for widefield/bulk applications as for a microscopeimplementation. Although the power density and associated photo-thermaldeformation may be lower when the IR and/or probe beam are spread over alarger area, the aggregate impact on the sample may still be detectable.For example, a detection system that integrates the small photo-thermaldistortion over a large area may have sufficient sensitivity even atsmall optical power densities. One way of achieving this for example isto use camera 210 or an alternate camera or array detector in received142. A camera or array detector can measure the small change inintensity and/or position of reflected or scattered light over aplurality of pixels and then coherently sum the aggregate impact on themotion/intensity of scattered probe light collected from a large area.Speckle patterns from rough samples, as discussed previously, can beanalyzed to determine a signal that is indicative of the photo-thermaldistortion of the sample and in turn the IR absorption of the sample.

The ability to obtain spectra from a wide area of a sample makes thephoto-thermal IR technique viable for use in a variety of applications,including material inspection, material composition analysis, evaluationof material treatments, hazardous materials detections, detection ofdefects and contaminants, and process control to name a few. Forexample, a PTP-IR/Raman system could be used to performing incomingmaterial inspection on bulk materials to verify composition against avendor or customer specification. Similarly, the PTP system can be usedto determine the material composition of an unknown material or checkthe composition against some predetermined targets. In this embodiment,the measured spectra can be a linear superposition of component spectrascaled by their relative concentrations. With chemometric or spectraldecomposition techniques, the PTP system can be used to deconvolve themixed spectra into component spectra and relative concentrations.

The evaluation of treated materials is also an important application.For example, many materials are exposed to chemical treatments to impacttheir wettability, flammability, resistance to ultraviolet radiation, orfor other purposes. A common analytical desire is to understand thedegree and uniformity of the uptake of the treating chemical into thebulk material. The PTP technique can be used to measure and map thedistribution of chemical treatments on materials, either in themicroscopy mode with highly focused beams, or via wide area measurementsor both. The PTP technique can also be used for hazardous materialsdetection, either with lab grade instruments in laboratories or in thefield via handheld instruments as described below in and shown in FIG. 9or alternately via a standoff detection mechanism.

The photo-thermal probe technology of the current application can alsobe applied to detection and identification of chemicals and materialswith a portable/hand-held device. FIG. 9A shows a simplified schematicof a miniaturized dual-beam device capable of performing IR spectroscopyin a reflection/backscatter mode. A compact tunable infrared source 900emits a beam of 902 that passes through beam combiner 904 to focusingoptic 906 for focus a beam 908 onto sample 910. Optional nosecone 912can provide a reference surface to position the miniaturized PTP deviceat a desired distance from the sample to ensure optimal focus. Thenosecone 912 may be removable or exchangeable to operate at differentstandoff distances, for example to position an IR beam such that it isfocused a fixed distance past the end of the nosecone. A probe lightsource 914 emits a beam 915 of UV, visible, near IR or other fixedwavelength radiation that passes through optional beamsplitter 916towards beam combiner 904 where the probe beam is reflected to the samefocusing optic 906 to focus the probe beam to a location on the sampleat least partially overlapping the focused IR beam. As discussedpreviously, the configuration of the beam combiner 904, IR and probebeams can be reversed, that is so that the probe combiner 904 transmitsvisible light and reflects IR.

The probe beam can be used for multiple purposes. It can be used toprobe the IR absorption of the sample due to the photo-thermal processdescribed previously, but it can also be used to generate Ramanscattering and/or fluorescence in the sample. For the IR measurements,as before, regions of the sample that absorb IR light will heat up,resulting in a photo-thermal distortion of the sample associated withthe absorbing regions. The photo-thermal distortion of the sample isdetected via the probe beam whose beam shaper and/or trajectory can bemodulated by the photo-thermal heating of the sample. Probe lightreturning from the sample (along with any resulting fluorescent and/orRaman scattered light excited by the probe beam) can be collected byoptic 906 or alternately a separate offset collection optic (not shown).In the embodiment, the probe light is collected by the same optic 906,the collected probe beam reflects off beam combiner 904 and into beamsplitter 916 where a portion of the beam passes through to receiver 914.The receiver can comprise any desired combinations of optical sensorsand spectrometers. Electronics 920 can be used to amplify/condition anddemodulate sensor and/or spectrometer signals to compute signalsindicative of IR absorption, Raman scattering and/or fluorescencesignals. The entire device may be surrounded with a ruggedized case 922to permit usage in harsh environments and/or workplaces where the devicemay be exposed to shocks, drops, and/or chemical exposure.

FIG. 9B shows a simplified conceptual diagram of the exterior of theminiature handheld PTP device measuring a sample 910. The view shows thetop view of the ruggedized case 922 with optional nosecone 912 inproximity of the sample 910. On the surface of the device there may beone or more buttons 924 for controlling various functions of the device.There may also be a display 926 that can display various controls,settings, measurement protocols, etc. used to configure a measurement.The display 926 can also show one or more spectra 928 obtained by IRand/or Raman spectroscopy and/or fluorescent measurements, or anindication of a particular chemical or composition of interest that hasbeen detected based upon the reflected spectrum. Spectra and/orfluorescent measurements can be used optionally in combination withchemometrics to identify an unknown material, producing a chemicalidentification 930 with the likely chemical composition of the sample.Alternately or additionally, the device can produce an analysis of thesample, for example providing a compositional makeup or percentagecomposition of one or more constituent chemicals. The device can also orinstead indicate whether the measured material meets specified standardsfor the target material and indicate whether a measured material passesor fails a test for constituent levels via indicators 932. The devicecan also or instead be used to measure the concentration of a specificcomponent, for example analyzing the percentage fat or protein or todetect the potential presence of a contaminant or hazardous substance.

Returning to FIG. 9A, heating beam source 914 can be an IR source. Forexample, IR source 914 can be a tunable quantum cascade laser (QCL)device. Block Engineering for example makes a miniature QCL module thatcan tune over 250 cm⁻¹ with a size of 46.5×32.5×20.5 mm, a size that canfit into handheld device. Multiple QCL modules can be combined to coverabout 5.4 μm to about 13.3 μm. In recent years both the average powerand tuning range of QCLs have been increasing, so it is expected thatbroader tuning ranges will be possible in the future with similarlysized or smaller sources.

Similarly, laser diodes suitable for sensing probe light sources areavailable from many vendors in a size suitable for a handheld device.Thorlabs sells many laser diode devices ranging from a few mW to morethan a watt over many wavelength ranges. For example, Thorlabs sells alaser diode part number LD785-SEV400 in a TO-9 can (9 mm diameter) with400 mW of optical power at 785 nm, sufficient for both sensing beammeasurements and as a source for Raman spectroscopy. Alternate packagesare available with fiber coupled laser diodes in many wavelengths.

Miniature collimating lenses are also available and complete laser diodemodules with collimation and drive electronics are also available in avariety of form factors from various vendors. IR and probe beamdiameters can be kept at the size of a few millimeters or less to enablethe use of small or miniature optics for beam combiner 904, focusingoptic 906, and beamsplitter 916. The beam combiner, focusing optic andbeamsplitter could each be 5 mm or less across, in an embodiment. Forexample, Edmund Optics makes a polarizing beamsplitter in a 5×5×5 mmsize and smaller beamsplitters and other optics can be fabricated on acustom basis. Visible laser diodes are similarly small, for example lessthan about 5 mm long and less than about 9 mm diameter. Thermoelectricor other cooling may be desired, but the space required for this can beon the scale of a few mm to a few cm depending on the optical powerlevel and associated heat load. Optical detectors, Raman spectrometers,and/or cameras are also available in miniature form factors with sizeson the scale from a few mm to a few cm. Power supplies, signalconditioning a computational electronics can also be miniaturized, downto the scale of a few cm for the functions required. As such, it ispossible to assemble an entire handheld photo-thermal probe device tofit into a package that is smaller than 125 mm across, in oneembodiment.

For higher power applications, it may be desirable to have a portion ofthe device remoted, for example with power supply, batteries, additionalcomputation resources, and/or larger IR and/or probe light sources. Inthis embodiment, the handheld unit may be coupled to the remote unit viaa wired connection or a wireless connection. A wired connection caninclude one or more cables and/or optical fibers to provide electricaland/or optical coupling between the handheld unit and the remote portionof the device. For example, separate IR and visible optical fibers maytransmit the IR and probe light to a handheld unit where the two beamsare combined as described in associated with FIG. 9A above.

The miniature PTP device can be used for a large number of applications,including but not limited to material inspection, material compositionanalysis, evaluation of material treatments, hazardous materialsdetections, detection of defects and contaminants, and process control.In the embodiment of standoff detection, where the unit is used toperform chemical analysis of a distant object, focusing optic 906 can bereplaced with a collimating optic so as to emit collimated IR and probebeams. Then the probe beam can measure the photo-thermal response andhence IR absorption from a distant object. For standoff detection, it ispossible to collect reflected/scattered light with the same collimatingoptic or alternately a separate optic, even with a standalone collector,for example an optical telescope fitted with an optical detector and/orcamera.

FIG. 10 shows an alternative embodiment of a PTP apparatus supportingmeasurements of samples in liquid, for example live biological cells.FIG. 10 is a modified version of FIG. 1B and similar objects arereferred to by the same reference numbers. The embodiment shown in FIG.10 enables measurements of IR spectroscopy and Raman and/or fluorescenceusing the same probe beam excitation and with the sample immersed inliquid. As with FIG. 1B and other similar figures, IR and probe beamsare combined together then sent through a focusing optic 120 to create afocused beam 122 to interrogate a sample. In this embodiment, the samplespecimen 1000 is mounted on a suitable substrate 1002 for measurementsin liquid, for example a glass slide or petri dish. A drop of liquid1006 covers at least a portion of the substrate 1002 and specimen 1004.The specimen and liquid drop are covered with an infrared transparentcover slip or window 1004. In a cover slip configuration, the cover slipis usually mounted on the sample substrate 1002 trapping a small amountof liquid between the substrate and coverslip. Alternately, thecoverslip may be replaced with an IR transparent window that isindependent of the sample substrate. For example, it may be separatestructure of a liquid cell and/or may be attached to or a part of focusoptic 120. In either embodiment, IR and probe light can pass through thecoverslip or window 1004 to impinge on specimen 1000 immersed in liquid.The probe light can pass through the sample substrate 1002, which chosento be substantially transparent at the probe wavelength. The probe lightand any resulting Raman or fluorescently scattered light 132 arecollected by collection optic 134 as in FIG. 1B and other similarfigures. The collected light can be collected as before by receiver 140and/or 142 and analyzed to produce signals indicative of IR absorption,Raman scattering and/or fluorescence. In embodiments, probe light andany Raman or fluorescent light can be collected in reflection via optic120.

The liquid can be any number of liquids depending on the purpose of theexperiment. For measurements on biological specimen, the liquid willnormally be an aqueous solution, for example a biological buffersolution, perhaps with various chemicals, drugs, toxins, etc. that arebeing studied in interaction with the specimen. The liquid can also bean oil or other liquid chosen to maintain the sample in an optimalenvironment, for example clean and/or not exposed to the atmosphere. Theliquid can also be used to provide an electrochemical environment, forexample for battery, fuel cell, or similar research. It can also be aliquid to test environment and/or chemical exposure to a sample orsubstrate. In the embodiment that the liquid absorbs IR radiation it canbe desirable to use a very thin layer of liquid to avoid excessive lossof optical power by the liquid. These concerns can be mitigated withtunable IR sources that facilitate modifications to the intensity of thelight. In embodiments, the intensity of the heating beam can beincreased whenever the source is tuned to an absorption band of theliquid to overcome signal loss to the liquid. Additionally oralternatively, background signals generated by the liquid absorption canbe removed by subtracting absorption signals related to regions of thesubstrate with no specimen from absorption signals from regions of thesample with specimen. The sample substrate 1002 can be placed on amanual and/or motorized translation stage 119 to allow measurements tobe performed at a plurality of locations on the sample substrate 1002 toenable mapping of IR absorption, Raman, and/or fluorescence overarbitrarily large areas of the sample.

FIG. 11 shows an alternative embodiment for optical measurements with asample immersed in liquid. In this embodiment, the heating beam 122illuminates the sample from below. This arrangement permits uses ofspecimen 1000 that are immersed in larger volumes of liquid, for examplein a petri dish 1100, perfusion cell, electrochemical cells or otherfluid immersion configurations in which it is difficult to invert theliquid. In the orientation shown in this embodiment, gravity keeps theliquid within the petri dish 1100 or other open container. To provide astable optical interface, an optional window 1102 can be placed at thetop surface of the liquid. The window may be part of an enclosed fluidcell can be spaced off the bottom of the petri dish with spacers,mounted to the housing or other structure of optic 134, or supported byother structure, for example on the sample translation stage 119 or anyother arrangement that provides a support for the window 1102. As withother embodiments, the probe light can be collected either intransmission mode with optical collector 134 or in reflection mode.

The measurement technique is not constrained just to mid-infraredwavelengths. The same benefit of sub-diffraction limited spatialresolution can be applied to longer wavelength measurements as well. Theratio of spatial resolution improvement can be much larger for longerwavelengths than for shorter wavelengths due to the difference in theirdiffraction limits.

FIG. 12 is a simplified schematic of an embodiment of a dual-beam,photo-thermal system and technique for sub-diffraction limited imaging.A source 1200 emits a beam of heating radiation 1202 that is reflectedoff one or more optional steering mirrors 1204 towards beam combiner1206 which reflects the heating beam towards optional mirror 1208 andinto focusing optic 1210. Focusing optic 1210 focuses the heating beam1212 to a spot on a region of interest 124 of sample 125. Because theheating beam can have a wavelength that is much larger than mid-IRwavelengths, the focused spot size in the embodiment shown in FIG. 12 isproportionately larger, for example about 1 mm in diameter. To performspatially resolved spectroscopy and chemical imaging below thediffraction limited size of the heating beam, a probe light beam is usedto read out the photo-thermal distortion of the sample resulting fromthe heating of the sample due to absorption of heating beam. Probe lightsource 126 emits a beam of probe light 127 that passes through beamcombiner 1206, is reflected off optional mirror 1208 and is focused byoptic 1210 onto the sample 125 in a region that at least partiallyoverlaps the region of the sample illuminated by the heating beam. Theprobe light is generally selected to have a smaller wavelength that theheating beam, for example in the UV, visible, near IR or even mid-IRwavelength range. The smaller wavelength of the probe beam allows it tobe focused to a much smaller diffraction limited spot, for example toless than 1000 nm, less than 500 nm, and even less than 200 nm. Afterinteracting with the sample the probe light can be collected bycollecting optic 134 and transmitted to receiver 140 that can compriseany number of optical detectors, spectrometers, cameras, etc. asdescribed previously associated with previous figures. Receiver 140 canalso include a detector for bulk/wide area THz spectroscopy. Heatingbeam source 1200 may be one of a number of THz sources in thatembodiment. For example, a number of vendors, for example Toptica andMenlo Systems make THz emitters based on femtosecond fiber lasers beingused to pump a ZnTe THz emitter to generate a beam of THz radiation. Thecompany M Squared makes a 30 nsec pulsed THz laser. EdinburghInstruments makes a continuous wave far-IR/THz laser with emission linesover 0.25-7.5 THz with power ranging from 10-150 mW.

An external chopper can be used to modulate the intensity of the heatingbeam. For example, Scietec makes a high speed chopper that can modulatea laser to over 100 kHz modulation rates. Alternately, various researchgroups and commercial manufacturers have demonstrated THz emitters basedon quantum cascade lasers (QCLs). Wang has demonstrated CW THz QCL with230 mW of optical power (article “High-power terahertz quantum cascadelasers with ˜0.23 Win continuous wave mode” published as DOI:10.1063/1.4959195). The Wang QCL is also a continuous wave device andcould be chopped as described previously. Burghoff et al. hasdemonstrated the use of broadband pulsed THz frequency combs to deliver5 mW over 60 THz laser lines (DOI:10.1038/nphoton.2014.85). LongwavePhotonics makes a THz QCL product line called “EasyQCL” that delivers 10mW of optical power, and can be pulsed up to 100 kHz, with emissionfrequencies in the range of 1.9-5 THz. Brandstetter et al. developed THzQCL with 200 nsec pulses, 10 kHz rep rate, almost 500 mW optical power(article “High power terahertz quantum cascade lasers with symmetricwafer bonded active regions,” DOI: 10.1063/1.4826943). Each of these maybe a suitable THz source for the heating beam of FIG. 12 , inembodiments, and can be used to take measurements depending on thepower, wavelength range, and pulse repetition requirements of a specificmeasurement. Because many of the THz QCLs operate at cryogenictemperatures, the THz QCL source 1200 may comprise a Dewar, a cryostator other cooling apparatus to maintain the QCL chip at a sufficientlylow temperature for operation.

FIG. 12 is shown in a configuration where the heating beam is reflectedby beam combiner 1206 and the probe light passes through the beamcombiner. A suitable optic for this configuration would for example bethe “Terahertz Visible Beamsplitter” model 20TZBS02-C available from theNewport Corporation. The relative orientation could be reversed for usewith an alternate beam combiner 1206 that reflected IR and transmittedheating beam radiation. The same type of probe beam may be used in thisembodiment as in previous embodiments. Specifically, this means that theprobe light source can (1) have a much smaller wavelength to providemuch better spatial resolution than the diffraction limit for theheating beam; and (2) the probe light source may be a light source thatexcites fluorescence and Raman scattering in the sample thus providingthe ability to perform collocated heating beam, Raman, and/orfluorescence measurements. It is also possible to provide an additionalor alternate light source in the form of an IR light source to providesub-diffraction limited measurements of IR absorption. THz measurementscan be combined with IR measurements and Raman and/or fluorescence usingthe same probe light beam, in one embodiment. The embodiment shown inFIG. 12 can also be combined with beam steering (FIGS. 2, 4, and 13 ),Raman and fluorescence (FIG. 3 ), measurements in vacuum (FIGS. 5-7 ),wide area measurements (FIG. 8 ), handheld measurements (FIG. 9 ), witha sample immersed in liquid (FIGS. 10 and 11 ).

Additional design advantages and options discussed below also applywhile using very large (1 mm or more) wavelengths. In each of theseembodiments the IR sources of and IR compatible beam combiners of FIGS.10 and 11 are replaced or augmented with one or more longer wave sourcesand longer wave compatible beam combiner as described above. Suchsystems can employ both IR and longer wave sources such that it ispossible to perform IR and longer wave spectroscopy of the same regionsof the sample for the purposes of correlative measurements.

FIG. 13 is a simplified schematic diagram of one aspect of a dual-beaminstrument according to an embodiment. In embodiments it can bedesirable for the probe beam is well aligned with the IR beam, asdescribed in more detail with respect to FIG. 4 . Even as the IR beam israpidly swept across a range of wavelengths, control of the positions ofthe two beams of the dual-beam system remains important. Many tunable IRsources have pointing errors that depend on wavelength and/or thethermal conditions of the IR source. This is illustrated schematicallyby center rays 1300 that leave the IR source 112 at different angles.Optional steering mirrors 202 and 204, as discussed earlier associatedwith FIG. 2 can be used to counteract these wavelength and temperaturedependent pointing errors. The mirrors 202 and 203 can be for example 2axis galvo mirror scanners, voice coil driven fast steering mirror (forexample sold by Optics in Motion), piezo driven tip/tilt stages,tiltable MEMS mirrors or other devices that can change the angle of amirror or a laser beam in response to an applied control signal, forexample an applied voltage. The steering mirrors can be used to adjustthe IR beam such that it is on a collinear or at least parallel path forall wavelengths within a given sweep range. This is illustratedschematically by central ray 1302 after mirror 202 where the threecentral rays 1300 have been corrected to once again be overlapping. Toachieve this overlap the demodulated probe signal is measured as afunction of the angular position of one or more steering mirror 200 and202. The demodulated probe signal, for example produced by analyzer 144,is an indicator of the product of the IR intensity absorbed by thesample 125 times the probe beam sensitivity to this IR absorbed light.The modulated signal is generally maximized when the peak intensity ofthe IR beam is substantially aligned with the peak intensity of theprobe beam, for example as shown in FIGS. 4A-C.

By measuring the PTP intensity as a function of steering mirrorpositions it is possible to sweep the IR beam emitted from the focusingoptic 120 over a plurality of angles 1304 to identify the tilt anglethat provide the best alignment of IR and probe beams. It is possible toconstruct a plot 1306 of the relative alignment by plotting the PTPintensity as a function of the mirror tilt angles (the plot shownindicates the mirror tilt angle in milliradians). This plot can begenerated for any specific IR wavelength or for a plurality ofwavelengths. From the plots, it is possible to select for example thepeak intensity or centroids of the response peak to determine optimalmirror tilt angles for a plurality of IR wavelengths.

Based on this result, appropriate control signals can be applied to themirrors 200/202 during the process of a spectroscopic measurement tokeep the dual beams co-aligned over a range of wavelengths, for exampleduring the process of generating a signal indicative of an infraredabsorption spectrum of a region of the sample. A look-up table of mirrorangles can be created that can be rapidly applied during the wavelengthsweep of the IR source. IR sources are available now that can be sweptover a range exceeding 1000 cm⁻¹ in less than a second and in fact inless than 100 msec. At 4 cm⁻¹ spectral resolution it may be desirable toadjust the mirror positions as much as 250 times per second for a 1second sweep over 1000 cm⁻¹. Fast steering mirrors from Optics in Motionhave a 3 dB bandwidth of 850 Hz, sufficient for the 4 cm-1 spectralresolution, especially for smoothly varying beam pointing errors are afunction of wavelength. Even a large step can be completed relativelyquickly, as the fast steering mirror can have a 5 msec step and settletime for a 1 mrad step. Scanning at 1000 cm⁻¹/sec would therefore incurat most a steering error for at most 5 cm-1 after a steeringdiscontinuity. Faster solutions for beam steering are also available.Tapos et al. (DOI: 10.1117/12.617960) demonstrated a piezo driven faststeering mirror with a bandwidth in excess of 2 kHz. Physike Instrumentsmanufactures a piezo tip/tilt fast steering mirror stage (model S331)with a resonant frequency of 10 kHz and a step response of 1.8 msec.Kluk et al at MIT Lincoln Labs have demonstrated a 2D fast steeringmirror with sub-msec step and settle time (published as DOI:10.1016/j.mechatronics.2012.01.008).

To provide good tracking of the heating beam position as a function ofwavelength, it is necessary to synchronize the position of the faststeering mirror while the heating beam source sweeps over a plurality ofwavelengths. To do this it is desirable that either the heating beamsource (e.g., an IR source) or controller driving the IR source emitsone or more sync signals to indicate the time at which the IR source isat a given wavelength, or after a fixed increment of wavenumber, forexample every 1 cm⁻¹ or 5 cm⁻¹ etc. as desired or required to providesufficient accuracy to control the fast steering mirror to correct thewavelength dependent beam pointing errors. Using the sync signal(s), acontrol waveform can be applied to the fast steering mirror to programthe right amount of correction at a plurality of wavelengths in the scanwhile the IR source wavelength is being swept. Note that it may bedesirable to apply a feed forward scheme to compensate for the finiteresponse time of the fast steering mirrors. That is that control signalmay be applied a short time before the IR laser is at a given wavelengthto allow the fast steering mirror to be at the correct tip/tilt anglewhen the IR source reaches the given wavelength.

Another way to create a calibration trajectory for the fast steeringmirror is to measure a series of IR absorption spectra on a sample as afunction of mirror tip/tilt angle. Then a computational algorithm cancompute the maximum amplitude for any given wavelength in the array ofspectra and then from that determine the position of the fast steeringmirror(s) than enabled the maximum amplitude. Then the fast steeringtrajectory is computed to be the set of tip/tilt values that producedthe substantially maximal response at each wavelength in the absorptionspectrum. The fast steering trajectory represents the set of mirrorangles for a plurality of wavelengths that produce substantially thebest alignment between the IR and probe beams. The fast steeringtrajectory can then be programmed to be supplied to the fast steeringmirror(s) synchronized with the wavelength sweep, as discussed above. Itmay also be desirable to apply an adjustable offset to the fast steeringmirror trajectory positions to compensate for temperature dependentfluctuations in beam pointing. For example, a sweep of the steeringmirror(s) may be performed at one or a small number of wavelengths todetermine an offset from an original reference position. This offset canthen be applied to other wavelengths in the steering mirror trajectorywithout the need to recalibrate the beam angle at each wavelength.Additionally, it is possible to calibrate the beam pointing errors withtemperature and then apply offset corrections based on a measuredtemperature of the IR laser and/or any associated mounting structure andoptical elements involved in the beam pointing. Using one or more of thesteps described above it is possible to create beam steering correctionsthat are synchronized to the wavelength of the IR source and to acquiresignals indicative of the IR absorption of the sample with very fastsweep speeds, for example in less than one second, or in less than 0.1second.

FIG. 14 shows an embodiment of a receiver module configured to supportPTP measurements with high sensitivity along with additional opticalpaths for Raman and/or fluorescence measurements. FIG. 14 is an expandedand somewhat modified version of a region of the region of FIG. 3showing the receiver module 142 and spectrometer 318. For items that arethe same as in FIG. 3 , the same numerical callouts are used and theassociated discussion from FIG. 3 apply as appropriate. In FIG. 14 theprobe beam 129 is shown starting with returning from the collectionoptics that enters the receiver module 142. The probe light 129 can beintercepted by optional pickoff mirrors 1400 and/or 1402 that can directa portion of the probe beam towards spectrometer 318 and/or towardsfluorescence detector 310. In FIG. 3 mirrors 302 and 304 were drawn tobe larger than the probe beam to have the option of directing all theprobe light to the spectrometer 318, fluorescence detector 312, and/oroptical detector 308. FIG. 14 shows the option of using smaller pickoffmirrors 1400 and 1402 to sample only a portion of the beam. There aretwo reasons for this. First, it provides one mechanism for providingparallel measurements of IR absorption and Raman and/or fluorescence bydirecting a portion of the collected probe light simultaneously to oneor more detector/spectrometer. But more significantly, the size andplacement of the pickoff mirrors is chosen to substantially amplify thesensitivity of the PTP technique. Photo-thermal distortion of the probebeam due to IR absorption tends to more significantly affect the extremerays that strike the sample with the largest angle. The central raysthat strike the sample closer to normal are more minimally affected. Assuch, blocking a portion of the central rays has no negative impact onthe PTP sensitivity and in fact can provide an improvement in signal tonoise ratio.

In some configurations, the signal to noise may be constrained bydetector noise in the measurement circuit optical detector 308 and/ordownstream signal conditioning or acquisition electronics. By blocking aportion of the probe beam that contributes only minimally to PTPsensitivity, the detection electronics can operate at higher gainwithout saturation. This increase in gain can boost the signal furtherabove the noise floor of the detector electronics and achieve highersignal to noise ratio. Pickoff mirrors 1400 and/or 1402 can be sizedspecifically to block a portion of the collected probe light that doesnot provide a significant fraction of the rays that provide the bulk ofthe PTP sensitivity and then reflect that light to one or both of theRaman spectrometer 318 or the fluorescence detector 312. One or bothpickoff mirrors can alternately be a dichroic mirror chosen toselectively reflect light of a target wavelength or over a specificwavelength range.

FIG. 14 also shows one embodiment of the beam shaping element 306described in FIG. 3 . In the embodiment shown the beam shaping elementhas an obscuration 1404 to block a portion of the probe beam with thepurpose of increasing the sensitivity of the PTP measurement. Althoughshown in the configuration with the Raman and/or fluorescence opticalpaths, the obscuration feature may be used in PTP only systems as well.For example, beam shaping element 306 may have a central obscuration,for example a round opaque region or a transparent annulus thatselectively blocks a portion of the probe beam that contributesminimally to the PTP sensitivity. Alternately it may block a portion ofthe light such that angular changes in the probe beam from sample IRabsorption will result in intensity modulations at the optical detector308. The optimal size of the central obscuration can be determinedexperimentally. For example, it is possible to insert an adjustable irisin the path of probe beam 129. The iris can be closed down whilemonitoring the strength of the PTP signal and/or the PTP signal to noiseratio. The iris can be closed down until there is a certain thresholdloss of PTP sensitivity or SNR. At this point it can be determined thatmost of the PTP sensitivity is delivered by extreme probe rays that areoutside the diameter of adjustable iris. By then creating a centralobscuration that blocks an area substantially similar to the currentiris diameter will provide substantially maximal PTP sensitivity and/orSNR. The central obscuration may be a simple disc for example that issubstantially opaque at the wavelength of the probe beam and issupported by thin support(s), e.g. so called “spider legs” that block aminimal amount of the desired extreme rays of the probe beam.Alternately the obscuration may be an opaque or reflecting mask on anotherwise transparent substrate, for example a metal or ink coating on aglass substrate. Alternately the beam shaping element may have a morecomplicated shape, for example a transparent annulus in an otherwiseopaque or reflective substrate. For example, Thorlabs sells annularaperture obstruction targets that have pinholes in a variety of sizeswith a central obscuration in the center of the pinhole. For thesetargets, it may be desirable to additionally shape the probe beam withfocusing optics (not shown) to size the beam appropriately relative tothe annulus and central obstruction.

Auto-Optimization

As can be seen by some of the embodiments of in this specification, itis possible for a PTP system to have a very large number of degrees offreedom that can require adjustment or optimization. For example, the IRpower, the probe beam power, the position of the focusing objective, theposition of the collection optic (if used), the relative alignment ofthe IR and probe beams to the focusing optic and the relative alignmentof the IR and probe beam to each other are all critical parameters thatneed to be carefully optimized. If not optimized it is possible to havea compromised signal to noise or even no signal. The large number ofdegrees of freedom could make it daunting or impossible for a user oflimited skill to perform successful PTP measurements. In variousembodiments, different techniques are used to automate the optimizationof the PTP technique. Some of these have already been described in part,for example associated with beam steering and IR/probe beam overlap.Some other techniques used for automated optimization are discussedbelow.

Avoiding sample damage. One challenge associated with PTP measurementsis optimizing the signal strength without damaging the sample. Tomaximize the PTP signal strength it can be desirable to increase thelaser power of the IR beam and/or the probe beam. The limit to which thepower can be adjusted depends on the absorptivity of the sample at theIR and probe wavelengths, along with the sample damage threshold. Thereare several ways to maximize the sensitivity while avoiding/minimizingsample damage. For example, it is possible to record the PTP sensitivityand/or SNR while increasing the optical power of at least one of the IRsource and the probe beam until either (1) the sensitivity/SNR reaches amaximum; and/or the (2) damage is observed in the sample. Sample damagecan be observed in a number of ways. For example, it is possible toperform a spectroscopic sweep by measuring the PTP signal across aplurality of wavelengths. In the case that damage is caused by IRabsorption, the damage will occur first at the strongest absorbingwavelengths. Sample damage can be detected by observing non-linearitiesand/or discontinuities in the spectra. For example, when some materialsreach a sufficiently elevated temperature they can undergo a glass torubber transition, resulting is a significantly larger thermal expansioncoefficient and hence a substantially larger change in the index ofrefraction with temperature do/dT. In practices this can mean thatstrong absorption peaks can look out of proportion to weaker peakscompared to the same measurements performed with lower IR laserintensity. So, it is possible to calculate for example the ratio inamplitude between a major peak and a minor peak. If that ratio changesabruptly at elevated IR laser powers, it can indicate that the sample isbeing damaged at the stronger absorption peaks and that lower laserpower should be used. Other samples may be melted and/or burnt atelevated temperatures resulting from excessive laser power. Theseeffects can also be detected via distortions and/or discontinuities inthe PTP IR absorption spectra. For example, one can measure a series ofspectra at increasing laser power. By plotting the amplitude of one ormore absorption peaks as a function of laser power the onset of damagecan be indicated with a nonlinear response of PTP signal intensityversus laser power, a drop in response, a discontinuity, and/or anirreversible change in the peak intensities or spectral quality.Detecting the onset of any of these conditions can establish a damagethreshold that can be used to constrain the laser power. It is alsopossible to use a video optical microscope with an image comparealgorithm. For example, a series of optical microscope images can beobtained with increasing laser power of either the IR beam or the probebeam power. Each subsequent image can be compared to one or any of theimages prior. An image compare algorithm can look for differencesbetween the images that are indicative of potential sample damage. Forexample, static images of the same locations measured below the damagethreshold should have substantially no differences except due to cameranoise. So, subtracting two images without intervening damage should showminimal difference. But after even subtle sample damage occurs, theimage subtraction will reveal a difference zone associated with thermalchanges to the sample. This process can be performed automatically byrapidly taking an image alternating with an exposure of increasing IRand/or probe beam power and damage thresholds can be determined for eachwavelength range. It is possible to then use these thresholds todynamically maximize the PTP sensitivity and/or SNR even as a functionof wavelength. For example, it is possible to increase the laser powerat weakly absorbing wavelengths and reduce the power at stronglyabsorbing wavelengths. Alternately, it is possible to dynamically reducethe probe beam power at strongly absorbing wavelengths and vice versa.

It is also possible to infer the sample damage threshold by observingthe thermal IR radiation emitted from the sample. As the temperature ofa sample increases, it emits an increasing amount of IR radiation andthe center wavelength of the emission changes in accordance withPlanck's law. From the intensity and/or center wavelength of the emittedIR radiation it is possible to estimate quantitatively the temperatureof regions of the sample that are illuminated by the IR and probe beams.The temperature can be compared against known or experimentallydetermined sample threshold temperatures that cause an unwanted materialchanges. For example, IR and probe beam intensities can be maintainedbelow material transition temperatures for example associated withglass-to-rubber transitions, melting, decomposition, desorption, etc.The sample IR temperatures from thermal radiation emission can bedetermined for example with an IR detector, an IR camera, and/or an IRspectrometer to measure the intensity and/or center wavelength of the IRemission. While this IR temperature measurement will in general beperformed with diffraction limited spatial resolution, it is stillpossible to use such measurements to infer the maximum temperaturewithin the illuminated volume.

FIG. 15 shows a simplified flow chart diagram for an embodiment of amethod to obtain dual-beam data while substantially minimizing oravoiding sample damage altogether. For convenience, throughout thedescription of FIG. 15 , the sensing beam is referred to as the “probebeam” and the heating beam is referred to as the “IR beam.” At 1500, thesample is illuminated with the IR beam at a wavelength Ai or over aplurality of wavelengths as described previously. The sample is alsoilluminated with the probe beam (1502) and the probe light is collected(1504) and analyzed/demodulated as described previously associated withFIG. 1B and related figures. At 1507, the probe response as a functionof wavelength or at a fixed wavelength can be analyzed to determine asignal indicative of the signal to noise ratio (SNR) of the PTPmeasurement. The IR and/or probe power can be iteratively adjusted (step1512) to substantially maximize the PTP signal strength. At 1508, thesample is optionally analyzed for signs of damage via one or more of themethods described above or similar/equivalent techniques for identifyingoverheating, burning, decomposition, and/or melting of the sample. Ifdamage is detected (the Y branch), the IR and/or probe power is reduced(1512) and 1500-1508 can be repeated. If no damage to the sample isdetected or 1508 is omitted, the probe response can be analyzed for anonlinear response (1510). By nonlinear response, it is meant, forexample, that for a given increment in probe power the PTP signal doesnot increase proportionately. For example, it may respond super-linearlyif the sample temperature has exceeded a glass transition or phasetransition causing a dramatic increase in thermal expansion coefficientand/or do/dT. The PTP signal may instead decrease with increasing laserpower if the sample damage results in a change in the materialconformation, or at the onset of dissociation, desorption, ablationand/or other damaging effects.

A nonlinear response can also be detecting by comparing peak ratios atdifferent IR absorption bands. If for example the peak ratio changesbetween the amplitude of two or more absorption peaks as a function ofincident laser power, it can indicate a chemical change in the sample.If any of these nonlinear responses is detected, the IR power can alsobe adjusted and steps repeated as with the sample damage condition. Ifno nonlinear response is detected and/or one or both tests are omitted,the system can produce a PTP-IR spectrum of one or more regions of thesample. All of these steps can proceed in an automated manner withoutneed for user intervention, in an embodiment.

Surface tracking. Because of the small depth of focus, PTP measurementson rough and/or highly curved samples may be a challenge as the PTPsignal can dramatically decrease as the sample surface moves out of theplane of focus of the probe beam. To overcome this issue, it is possibleto dynamically adjust the focus of the probe beam in response to thesample profile. FIGS. 16A, 16B, and 16C relate to this issue. FIG. 16Ais based on FIG. 1 and common elements share the reference numerals fromFIG. 1 and the associated description applies to those components.

As before, IR and probe light is focused on sample 125 and it is desiredto measure the PTP response is measured at a plurality of sample oflocations. To accommodate for non-flat surface profile of sample 125, ameasurement is made of the position of the sample surface under thefocusing optic 120 to determine a relative distance Z₀ from somereference position within the system. This can be achieved using theprocess shown in FIG. 16B. At 1602, the sample is positioned at an XYlocation corresponding to a region of interest. At 1604, the relativesample height Z₀ is measured, using for example one or more of theheight measuring methods described in the next paragraph. The relativesample height Z₀ can either be relative to a fixed point within thesystem, for example a reference location of the focus optic 120 orrelative to a previous measurement of sample height at a prior referencelocation. This process is repeated as often as desired at as many XYlocations as desired. The XY locations may for example be a linear arrayand/or a regular grid of XY locations and/or a select set of XYlocations over specific regions of interest. This sample distance Z₀ ismeasured at a plurality of locations, for example across a linearprofile or at an array of X, Y locations on the sample to obtain one ormore ID surface profiles Z₀(X) (1614) and/or a 2D surface contourZ₀(X,Y).

After measurements of the relative sample height at a plurality of XYlocations, the resulting measurements form a profile or surface (1606)that represents the variation in height of the sample. A single slice ofa surface profile 1614 for example is shown in FIG. 16C. Once thedesired number of XY locations are measured to create a sample heightprofile, this profile can be used to make optimal measurements of thePTP signal. Specifically, the sample is moved to a desired XY location(1608), the focus optic is moved to the height Z₀ recorded during step1604 (1610), and the PTP signal is measured (1610). These steps arerepeated for as many XY locations as desired. Then a PTP image can becreated from the measurements acquired at the optimal focus positionsZ₀.

This relative sample distance Z₀ can be measured in any number of ways,for example with a distance measuring interferometer, a capacitancesensor, by optical triangulation, by confocal optical microscopy, by awhite light interferometer, optical autofocus, or other similar meanssensitive to the surface position. Some example arrangements andrelevant data are shown in FIGS. 17A, 17B and 17C as well as FIGS. 18A,18B, and 18C. FIG. 17A shows an example of performing the measurementusing a Mirau interferometer built into a specially constructedSchwarzchild objective. Light 1700 is directed through the centralaperture 1702 of a primary mirror 1704 where it strikes a smallsecondary mirror 1706. The light from the secondary mirror is reflectedto the bottom surface 1707 of the primary mirror 1704 where it is thenfocused on a region of interest 1710 of a sample 1712. A thin platebeamsplitter 1708 may be placed between the secondary mirror 1706 andthe sample 1712. A portion of the incident light reflects off thebeamsplitter 1708 and is reflected up to a reference reflector 1714which can optionally be coincident with the back side of secondarymirror 1706. Light reflected from the reference reflector travels backto the beamsplitter 1708 where a portion of the light is reflected backalong the incident beam path. This light combines with light reflectedfrom the sample surface. These two beams, one from the sample and onefrom the reference reflector combined to create an optical interference.These two beams can be collected and measured by a suitable opticaldetector (not shown). If the optical intensity at the detector ismeasured as a function of the position of the Schwarzchild objective ismoved vertically as shown by arrow 1714 (or equivalently the height ofsample 1712 is varied), an interferogram 1714 will result.

The detailed shape of the interferogram will depend on the opticalbandwidth of the light source used to illuminate the sample. In general,it is preferable to measure with a shorter wavelength, e.g., thewavelength of a probe beam, for example in the UV or visible wavelengthrange. For a narrowband source, the interferogram 1714 will have agenerally sinusoidal shape at it is possible to interpolate the relativeposition of the sample to a nanometer or better. It is also possible touse a less coherent source, for example an LED or a white light source.In this embodiment, as shown in FIG. 17B, the interferogram will have anoscillating envelope with a peak 1716 corresponding to the point ofgreatest phase coherence and hence greatest interference. This so called“centerburst” can be used to identify an absolute position at which thesurface of sample 1712 surface and the reference reflector 1714 areequidistant from the beamsplitter 1708. By measuring the position of thepeak 1716 of the interferogram at a plurality of X or XY locations onthe sample, it is possible to create a map of the sample height, i.e. asurface profile 1718 in FIG. 17C to use to obtain optimal measurementsof the IR absorption via the PTP technique. Thus, it is possible tomeasure the position of the peak or center 1716 of the centerburst atplurality of locations on the sample to obtain a surface profile 1718and then follow this surface profile 1718 during subsequent measurementsof a PTP signal. It is also possible to interleave the sample heightmeasurements and PTP measurements. For example, the sample height can bemeasured at each sample XY location just prior to the PTP measurementand the relative separation between the focusing optic and the samplesurface can be driven to the Z₀ position determined just prior to thePTP measurement.

Note that the Mirau interferometer setup shown in FIG. 17A may also beused in combination with the probe beam to measure IR absorption of thesample. For example, the IR and probe beams can directed through theSchwarzchild objective with the integrated Mirau interferometer in theposition of focusing optic 120 in FIG. 1B, and the resulting interferinglight can be detected and measured by receiver 142 in FIG. 1B. Theamplitude and/or optical phase of the interfering probe light can beindicative of the IR absorption. For example, thermal expansion of thesample due to IR absorption can cause a deformation of the reflectingsurface that will result in an optical path difference between the lightreflected from the sample surface versus the beamsplitter 1708. Thisdifference can result in a change in the intensity of the light detectedat receiver 142 in FIG. 1B. The detected intensity can be demodulated asdescribed previously, by measuring an amplitude of the modulation of theintensity at a frequency corresponding to the modulation of the IR beamor a harmonic thereof. The beamsplitter 1708 can also be removable suchthat it is not in the beam path for PTP measurements. Beamsplitter ispreferable constructed such that it is highly transmissive at IRwavelengths and partially reflective at the probe wavelengths and/or thewavelengths employed for sample height measurements.

FIG. 18A shows an alternative embodiment for rapidly measuring theprofile of a sample surface in advance of a PTP measurement. FIG. 18A issimilar to FIG. 17A and descriptions associated with FIG. 17A apply asappropriate. In this embodiment, a light beam 1800 is directed throughthe same center aperture 1702 of primary mirror 1704 as in FIG. 17A Theincoming beam strikes secondary mirror 1706 and is reflected to thebottom surface of primary mirror 1704 and then directed to a region ofinterest 1710 of sample 1712. The returning beam 1802 follows a paththat is similar to the incoming beam, but on the opposite sides of thesecondary mirror 1706 and primary mirror 1704. After passing through thecentral aperture the outgoing beam is reflected off mirror 1804 towardsdetector 1806. Mirror 1804 can be a removable mirror, an automaticallyrotatable mirror, or a beamsplitter. Detector 1806 may be a simpleintensity detector or may also comprise a position sensitive detector.In either embodiment, as the Schwarzchild objective is moved up or downrelative to the sample 1712 as indicated by arrow 1714, the positionand/or intensity of the beam on 1806 can change. The system canautomatically adjust the objective position to rapidly drive theobjective to a height that for example gives the maximum signal on anintensity detector or reflects the light to a desired reference locationon a position sensitive detector.

A related way of finding the surface height is by monitoring the DCintensity of the probe beam as a function of the objective height. Forexample, the probe beam that is reflected/scattered from the sample isdirected towards a receiver (e.g., receiver 142 of FIGS. 1B and 16A)that can comprise an optical detector. While the AC component of thissignal is analyzed by analyzer 144 in FIG. 1B, the DC component may alsobe used to determine the relative position of the surface tosubstantially maximize the PTP signal. FIG. 18B shows an example ofconcurrent measurements of the DC intensity 1808 of the probe beam andthe PTP signal 1810 as a function of the relative position (in μm) ofthe focusing optic (120 in FIGS. 1B and 16A). While the DC signal 1808has a much broader depth of focus, the centroid of the DC signal curve1808 has substantially the same center as the PTP curve 1810. Theadvantage of using the DC signal is that it is many, many times strongerthan the PTP signal and hence can be measured much faster. In addition,measuring the DC signal requires no knowledge of the sample, whereas toobtain a PTP signal it is necessary to tune the IR laser to an absorbingwavelength of the region of interest 1710 of the sample 1712. This canbe problematic when the sample is an unknown material as it will not beclear what wavelength should be used to optimize the measurement. So,using the probe DC signal to determine the surf ace height first removesa degree of freedom and ensures that the PTP measurement is started at asubstantially optimal focus height. Note that it also not necessary tomeasure a whole curve as in FIG. 18B. FIG. 18C illustrates an efficientway to quickly find the peak of the probe DC signal (or in fact the PTPsignal) by sampling only a few points. FIG. 18C illustrates performing ameasurement of the DC signal at a select few points, in this example 3points, and then using these three points to fit a parabola or othercurve 1814 to the measured data. (A parabola can be fit to 3 pointsalgebraically with no need for iterations.) From the fit it is possibleto determine the peak position Z0 1816 of the probe DC signal and thenuse this Z₀ position to perform PTP measurements at this location on thesample. And as described above, this process can be repeated at aplurality of locations on the sample to make a map of the surfacevariation across regions of interest of the sample to enable optimal PTPmeasurements at those sample locations.

Under the embodiments described, it is possible to use the PTP device toperform high quality measurements of IR absorption spectra of materialswith a signal to noise ration (SNR) of greater than 20 with ameasurement time of 1 second per spectrum or less, while achieving awith a spatial resolution well below the diffraction limit of mid-IRlight, specifically less than 1 μm. FIG. 19A shows an example of asingle spectrum obtained with 1 second acquisition time over thewavenumber range from 765 cm⁻¹ to 1868 cm⁻¹ with a SNR of 21 on a sampleof epoxy. This spectrum was obtained using a Block LaserTune QCLoperating at a repetition rate of 96.5 kHz in reflection mode (as shownin FIG. 3 ). The probe beam was collected with an amplified Thorlabssilicon photodetector with variable gain, and the PTP signal wasdemodulated with a Zurich Instruments HF2LI lock-in amplifier using theQCL pulse signal as a reference. The PTP signal was demodulated with thelock-in using a 1 msec time constant. The Block QCL was set to operatein continuous sweep mode with 1 second spectrum time and 0.3 sec time toreturn to the starting wavelength of the next spectrum. Data wastransferred from the lock-in every 0.5 cm⁻¹. No additional signalprocessing has been applied, i.e. no averaging, smoothing, etc. and thespectrum has not been corrected for the QCL power background. Thespectrum was acquired with the sample and IR beam path in open air, soIR absorption bands associated with CO₂ and water vapor absorption canbe present. These absorptions can be largely eliminated by enclosing thesystem in an enclosure that is purged with dry nitrogen, an inert gas,and/or exposed to a desiccant.

The signal to noise was calculated by determining the ratio between thepeak height for the maximum absorption band divided by the RMS noise ina non-absorbing region of the spectrum. Using these operatingconditions, it is possible to perform a hyperspectral array of 50×50PTP-IR spectra in less than 1 hour over a region of more than 1000 cm⁻¹with a spectral resolution of better than 1 cm⁻¹. Faster hyperspectralarrays can be achieved with faster spectral sweep speeds, for exampledown to 100 msec, using bi-directional sweeps, i.e. acquiring data inboth the forward and reverse wavenumber sweeps. Higher signal to noisecan be achieved by averaging multiple spectra, using longer sweep timeswith longer lock-in time constants, and/or using longer lock-in timeconstants at the cost of spectral resolution. FIGS. 19B and 19C showsthe averages of 10 spectra and 244 spectra respectively measured in aplurality of locations on epoxy regions from a hyperspectral array asdescribed above. The SNR for the 10 spectra is 76. It is equivalentlypossible to obtain a similar or better SNR from a 10 second acquisitiontime at a single point on a sample. The SNR for the spectrum in FIG. 19Cis 269. Note that these spectra were obtained on the block face (topsurface) of a microtomed sample in reflection. Thin sectioning was notrequired. Note also that the spectra, while these PTP measurements wereobtained in reflection, they are substantially free of reflection-basedspectral distortions that common to measurements performed byconventional FTIR spectroscopy measurements performed in reflectionmode.

Depth profiling. It is also possible to use the PTP technique forsub-surface depth profiling. The PTP signal primarily comes from a smallregion centered around the focus of the probe beam, from a volumeenclosed within the depth of focus of the PTP beam. A confocal aperturecan be placed in the PTP beam path to block light that does not passthrough this depth of focus volume. The IR absorption through the depthof a sample can therefore be determined by scanning the focus of theprobe beam relative to the sample surface. Because the PTP apparatus canalso perform confocal Raman measurements using a Raman spectrometer inor connected to one or more of the receiver modules, this apparatusenables simultaneous or sequential IR and Raman depth profiling on thesame regions of the same sample. This provides an unparalleledcapability to perform correlative measurements between Raman and IRspectroscopy.

FIG. 20 shows a simplified embodiment combining PTP measurements withthermal desorption mass spectrometry. FIG. 20 is similar to FIG. 1B andFIG. 2 , and where common callouts are used, the discussion associatedwith FIGS. 1B and 2 applies as appropriate. In FIG. 20 the IR beam 122and/or probe beam 127 may be supplied with sufficient intensity tothermally desorb and/or ablate material 200 from the surface of sample125. Desorbed/ablated material 2000 can be captured by a capillary 2002or other collection device and transferred in the direction of arrow2004 to the inlet of a mass spectrometer 2006. The desorbed/ablatedmaterial may also be ionized prior to introduction into the massspectrometer, for example using atmospheric pressure chemical ionization(APCI), electrospray ionization (ESI), or any other desired ionizationtechnique. Other fragmentation/separation techniques may be applied ascommon with mass spectrometry. In the end, the desorbed/ablated samplematerial can enter one or more mass analyzer to determine thedistribution of molecular masses/mass fragments in the desorbed/ablatedmass stream. From this it is possible to determine the chemical makeupof the sample using a diverse set of techniques, including IRabsorption, Raman, fluorescence, and mass spectrometry.

FIG. 21 is an illustration of a hyperspectral cube 2100 according to anembodiment. Hyperspectral cube 2100 is a variation on a conventionalsystem for depicting sensed optical data. Hyperspectral cube 2100includes an x axis, a y axis, and a λ axis. The x and y axes correspondto positions on a sample of interest. In embodiments, a dual-beammeasurement is taken of the sample at a particular x and y Cartesiancoordinate. That location can be probed for optical properties at aparticular wavelength. The cube is formed by probing for opticalproperties at each of a series of wavelengths along the λ axis, orequivalently at a series of wavenumbers

Compared to conventional systems, the dual-beam system described aboveprovides for significantly higher resolution along both x and y axes.Whereas conventional IR systems might have resolution down to on theorder of 1 μm, the systems described herein can have resolution down tohundreds nanometers. By using low wavelengths or interference asdescribed in FIG. 4 , the resolution can be enhanced even further toprovide for resolution down to 10 nm or shorter when combined withAFM-IR measurements. Furthermore, this improvement in spatial resolutionalong the x and y axes does not correspond to any decrease in resolutionin the λ axis, and the same wavelengths of interest can be probed withthe heating beam to sense IR spectroscopy data or Raman spectroscopy.

Each layer 2102 within the spectral cube 2100 corresponds to anabsorption of a particular wavelength as a function of position. Asshown in layer 2102, absorption at a particular wavelength followsseveral bands throughout the x-y plane in that particular layer. Layerscan indicative of a material of interest, for example where the materialof interest has strong or weak absorption characteristics at a certainwavelength. Likewise, each plane 2104 in the y-λ plane can be used topoint out bands in the x axis. Similar planes could be constructed inthe x-λ plane to sense bands in the y axis. Each column within thespectral cube 2100 contains spectral profile 2106. That is, at anyparticular position, the absorption spectrum of the material at eachwavelength extends along the λ axis. In the illustration of FIG. 21 ,the data used is from an AFM-IR measurement, but all of the techniquesdescribed above apply similarly for PTP measurements.

FIG. 22 shows a plurality of such spectra, three of which are numbered2106A, 2106B, and 2106C. These spectra were acquired in a line spectralarray, i.e. a series of spectra accumulated at successive locationsacross a sample in a linear array. The curves shown in FIG. 22 areabsorption as a function of wavenumber. The peaks in each linecorrespond to molecular absorption peaks. It is possible to use such alinear spectral array to identify the chemical composition of materialcomponents and the key absorption peaks associated with the materialcomponents. To determine the location of a material of interest, anoperator of a dual-beam system could determine the spatial locationwhere one or more peaks of interest shown in FIG. 22 . It is thenpossible to use the knowledge of the absorption peak locations toperform chemical imaging in the x-y plane at absorption peaks ofinterest, as shown in FIG. 23 . Such information can also be obtainedfrom the hyperspectral cube 2100 in FIG. 21 , i.e. identifyingabsorption bands and then determining where it would be beneficial toconduct a scan and determine which regions absorb at thosewavelengths/wavenumbers.

FIG. 23 is an example of a selection of a λ or wavenumber that can beused to detect the location of a substance. As shown in the absorptionspectrum 2302, peaks P1, P2, P3, and P4 exist corresponding to amaterial of interest. An operator can select one of these peaks forfurther analysis. Here, peak P3 has been selected. A sample can bescanned for absorption at that peak, as shown at 2304. Bands indicativeof high absorption within that wavelength correspond to areas of highconcentration of the material of interest (or vice versa, depending onwhether an absorption peak or transmission peak is selected).

In embodiments, the absorption spectrum 2302 can be taken from adatabase of known materials, or alternatively it can be measured fromthe sample or a reference material. Once a peak is selected, absorptioncan be measured using a scanning mode across the sample, or it can bedetermined according to the “snapshot” or image capture embodimentsdescribed in more detail above.

FIG. 24 depicts examples of three x-y scans corresponding to particularwavelengths/wavenumbers First x-y scan 2402 is an optical view of asample. Second x-y scan 2404 is an absorption spectrum at 1760 cm⁻¹,which has been identified as a wavelength of interest for detecting asubstance. Third x-y scan 2406 is an absorption spectrum at 1666 cm⁻¹,which has also been identified as a wavelength of interest for detectingthe substance. The comparison of second x-y scan 2404 and third x-y scan2406 can show the position of material that absorbs at both wavelengths,just 1760 cm⁻¹, and just 1666 cm⁻¹. Based on this comparison, thelocation of specific materials that absorb at each wavelength can beresolved. The embodiments described herein are exemplary. Modifications,rearrangements, substitute processes, alternative elements, etc. may bemade to these embodiments and still be encompassed within the teachingsset forth herein. One or more of the steps, processes, or methodsdescribed herein may be carried out by one or more processing and/ordigital devices, suitably programmed.

Depending on the embodiment, certain acts, events, or functions of anyof the method steps described herein can be performed in a differentsequence, can be added, merged, or left out altogether (e.g., not alldescribed acts or events are necessary for the practice of thealgorithm). Moreover, in certain embodiments, acts or events can beperformed concurrently, rather than sequentially.

The various illustrative logical blocks, optical and control elements,and method steps described in connection with the embodiments disclosedherein can be implemented as electronic hardware, computer software, orcombinations of both. To clearly illustrate this interchangeability ofhardware and software, various illustrative components, blocks, modules,and steps have been described above generally in terms of theirfunctionality. Whether such functionality is implemented as hardware orsoftware depends upon the particular application and design constraintsimposed on the overall system. The described functionality can beimplemented in varying ways for each particular application, but suchimplementation decisions should not be interpreted as causing adeparture from the scope of the disclosure.

The various illustrative logical blocks and modules described inconnection with the embodiments disclosed herein can be implemented orperformed by a machine, such as a processor configured with specificinstructions, a digital signal processor (DSP), an application specificintegrated circuit (ASIC), a field programmable gate array (FPGA) orother programmable logic device, discrete gate or transistor logic,discrete hardware components, or any combination thereof designed toperform the functions described herein. A processor can be amicroprocessor, but in the alternative, the processor can be acontroller, microcontroller, or state machine, combinations of the same,or the like. A processor can also be implemented as a combination ofcomputing devices, e.g., a combination of a DSP and a microprocessor, aplurality of microprocessors, one or more microprocessors in conjunctionwith a DSP core, or any other such configuration.

The elements of a method, process, or algorithm described in connectionwith the embodiments disclosed herein can be embodied directly inhardware, in a software module executed by a processor, or in acombination of the two. A software module can reside in RAM memory,flash memory, ROM memory, EPROM memory, EEPROM memory, registers, harddisk, a removable disk, a CD-ROM, or any other form of computer-readablestorage medium known in the art. An exemplary storage medium can becoupled to the processor such that the processor can read informationfrom, and write information to, the storage medium. In the alternative,the storage medium can be integral to the processor. The processor andthe storage medium can reside in an ASIC. A software module can comprisecomputer-executable instructions which cause a hardware processor toexecute the computer-executable instructions.

Conditional language used herein, such as, among others, “can,” “might,”“may,” “e.g.,” and the like, unless specifically stated otherwise, orotherwise understood within the context as used, is generally intendedto convey that certain embodiments include, while other embodiments donot include, certain features, elements, and/or states. Thus, suchconditional language is not generally intended to imply that features,elements and/or states are in any way required for one or moreembodiments or that one or more embodiments necessarily include logicfor deciding, with or without author input or prompting, whether thesefeatures, elements and/or states are included or are to be performed inany particular embodiment. The terms “comprising,” “including,”“having,” “involving,” and the like are synonymous and are usedinclusively, in an open-ended fashion, and do not exclude additionalelements, features, acts, operations, and so forth. Also, the term “or”is used in its inclusive sense (and not in its exclusive sense) so thatwhen used, for example, to connect a list of elements, the term “or”means one, some, or all of the elements in the list.

Disjunctive language such as the phrase “at least one of X, Y or Z,”unless specifically stated otherwise, is otherwise understood with thecontext as used in general to present that an item, term, etc., may beeither X, Y or Z, or any combination thereof (e.g., X, Y and/or Z).Thus, such disjunctive language is not generally intended to, and shouldnot, imply that certain embodiments require at least one of X, at leastone of Y or at least one of Z to each be present.

Unless otherwise explicitly stated, articles such as “a” or “an” shouldgenerally be interpreted to include one or more described items.Accordingly, phrases such as “a device configured to” are intended toinclude one or more recited devices. Such one or more recited devicescan also be collectively configured to carry out the stated recitations.For example, “a processor configured to carry out recitations A, Band C”can include a first processor configured to carry out recitation Aworking in conjunction with a second processor configured to carry outrecitations B and C.

Any incorporation by reference of documents above is limited such thatno subject matter is incorporated that is contrary to the explicitdisclosure herein. Any incorporation by reference of documents above isfurther limited such that no claims included in the documents areincorporated by reference herein. Any incorporation by reference ofdocuments above is yet further limited such that any definitionsprovided in the documents are not incorporated by reference hereinunless expressly included herein.

For purposes of interpreting the claims, it is expressly intended thatthe provisions of Section 112, sixth paragraph of 35 U.S.C. are not tobe invoked unless the specific terms “means for” or “step for” arerecited in a claim.

While the above detailed description has shown, described, and pointedout novel features as applied to illustrative embodiments, it will beunderstood that various omissions, substitutions, and changes in theform and details of the devices or methods illustrated can be madewithout departing from the spirit of the disclosure. As will berecognized, certain embodiments described herein can be embodied withina form that does not provide all of the features and benefits set forthherein, as some features can be used or practiced separately fromothers. All changes which come within the meaning and range ofequivalency of the claims are to be embraced within their scope.

We claim:
 1. A method for analyzing a sample, the method comprising: a,illuminating a region of the sample with a light beam of infraredradiation; b, illuminating at least a sub-region of the region of thesample with a probe light beam having a shorter wavelength than thelight beam of infrared radiation; c, analyzing probe light collectedfrom the sample to obtain measurements indicative of infrared absorptionof the sub-region of the sample; and d, collecting fluorescent lightfrom the sample in response to stimulation of the sample by the probelight beam.
 2. The method of claim 1 wherein a-c are repeated at aplurality of wavelengths of the light beam of infrared radiation.
 3. Themethod of claim 1 further comprising generating a spectrum of infraredabsorption by the sub-region of the sample.
 4. The method of claim 1wherein a-d are repeated at a plurality of locations on the sample. 5.The method of claim 4 further comprising generating a map of infraredabsorption of the plurality of locations on the sample.
 6. The method ofclaim 5 further comprising using fluorescent light collected from thesample to create a map of fluorescent response of the sample.
 7. Themethod of claim 1 further comprising creating overlapping mapsindicative of infrared absorption and fluorescent response of the samplefrom the measurements indicative of IR absorption and fluorescent lightcollected from the sample.
 8. The method of claim 1 further comprisinganalyzing probe light to obtain measurements indicative of Ramanscattering of the sub-region of the sample.
 9. The method of claim 1wherein the measurement indicative of infrared absorption and thefluorescent light collected from the sample are performed onsubstantially the same region of the sample.